Yes, as everyone is stating, a spark igniting hydrogen gas could cause an explosion, causing injury from small parts, sulfuric acid or both. Very nasty. Hydrogen gas is a byproduct of electrical energy created from a chemical reaction of lead plates submerged into the acid and water. Sparks can happen internally, too. Like when the lead plates warp from water evaporating out. This is why some are cynical about 'sealed' cells on an unmaintainable lead acid battery. Spark wouldn't necessary come from jumper cables or charger cables in this case, though, but rather simply attempting to start the vehicle.
It seemed like there were a couple of specific questions you had that I will attempt to answer.
Don't attach negative clip to the neg terminal on the dead battery unless you want the car's wiring removed from the battery. Attach to clean, unpainted part of frame or engine block bolt. This will keep any possible spark or spark arcing away from the potential gas. The contradictory part of whatever website you wrote from in your original question post was obviously either a mistake or the part where it spoke of attaching to the negative terminal was describing the process of jump-starting from a charged battery to a dead one. In that case you always attach dead-side first, with negative clamped to frame or engine, but the positive and negative attached to their perspective terminals on the charged battery (on the other side of cables). Again, the exception is if you remove the car's wiring.
The reason you go from dead to charged is to reduce the risk of spark. You can't take the risk of spark away completely, which is why it's better to attach the negative clip away from neg battery terminal. Same principal with a charger. Don't plug into power until clips are clipped. Just like going from dead to charged when charging from another battery, don't plug in charger until clips are on.
Another question I think you had was if there are other types of automotive batteries that don't have this risk. The answer is that unless it is lithium ion (or similar) batteries found in hybrid/electric cars or sometimes found in an emergency kit, they will ALL be some kind of lead acid type, even though there are a few different kinds of lead-acid batteries in use. They all hold the same risk.
Here is a link to a credible source regarding a study that took place with lead-acid batteries and explosions. It also includes all the other information I stated above about safety measures like hooking the negative to frame or block instead of terminal and the reasons for these safety measures:
Edit: additional info for an additional question. Should car wiring be removed from terminals before charging the battery?
This is a double-edged sword so you'll have to ultimately decide for yourself.
When working on the electrical system for repairs, you should remove the negative cable off of the terminal, anyway, so you don't accidentally short a circuit and damage an electrical component. The problem with cutting the car's circuit is that many ECU/PCU units have adaptive learning to help it control everything and disconnecting the circuit will force it to 'relearn' timing, shifting, etc.** This is due to Keep Alive Memory (KAM), a chip that stores information, which will erase when disconnecting a car's circuit from it's electrical source. The problems can vary from simply waiting for the 'relearn' process to even having to perform manual resets with a scan tool in order to get certain functions working again. Worst-case scenarios can require replacement of ECU altogether in order to get certain functions working again. Worst-case scenarios are usually only possible with early 2000's and later. (90's won't have these problems but still may need to relearn engine cycles, tranny shifting, etc.). There are pocket lists that specify specific problems with specific models out there, but you probably won't find one big, conclusive list for all problems with all models, so research on your specific model is always a good idea. There are KAM savers that plug into a cigarette lighter and use a 9-volt battery to Keep the KAM chip from erasing info while the battery is disconnected, but this will only be helpful if you can complete the repairs before the 9-volt battery is drained, maybe 30 minutes, +/-. Obviously, this won't help while charging the battery. So that is the downside to removing the cables from the battery to charge it. That and the risk of spark increases. Also, if you remove the cables, you have to connect the charger's ground clamp directly to the negative terminal on the battery instead of the frame or block. My previous edit stated otherwise and, after rereading it, I realized the error in that statement.
The benefit of removing the cables is that it will be impossible for a power surge to damage anything electrical-related in your car. Since the battery charger is plugged into A/C current, surges can obviously cause electrical damage. That's why I say the answer is a double-edged sword, because there are risks either way you go. If it were me, I'd keep the cables connected while charging, but use a GOOD surge protector for the charging device. By good I don't mean a $10 cheapie. If you keep the cables connected, follow this process: Plug the charger's positive clip onto the positive battery terminal. Then plug the negative clip onto the frame or engine block as far away from the battery as possible. Then plug the charger into a good surge protector, followed by plugging the surge protector to the wall outlet. Then turn the charger's switch on. Lastly, turn the surge protector's switch on. If it doesn't have one, buy a good one that does have a switch. A few final points: It's safer to use a lower amperage charger, between 2-6 amps, than to use a faster charger. It's bad for your charging system if you don't charge the battery completely, so be sure the charging process is complete. Lastly, I believe it IS necessary to have the car's ground cable connected to the negative battery terminal for the charger to charge it IF you clip the charger's ground to the frame or block. If you want the cable removed, you'll have to connect The clip directly to the battery. I'm sorry if reading my previous edit was misleading. If you decide to remove the cables, the negative cable ALWAYS goes first to prevent accidentally creating a complete circuit with a tool, jewelry, etc. Always use eye protection while working with the battery. I hope this answered your questions to your satisfaction.
**This adaptive learning can throw some people off after improving their vehicle performance. For example, say you complete a post-coil(s) ignition system overhaul. This will improve spark efficiency, which will subsequently advance timing. The ECU is used to the more retarded timing and will have to learn the improved cycling. This can fool some people into thinking they did something wrong but, if you just drive it around for a while, it will eventually relearn the more efficient timing. This process can vary widely, from 25 to 100+ miles. It may even take a few days of separate start-ups. During the process, you may experience erratic idling and acceleration, alarming exhaust, etc. Unless it's extremely obvious there's a problem with the work that was done or the problems don't go away, it should clear up on its own.