Molding is made hollow so it will conform to the contact surfaces better, which makes a better seal. This is really pretty common practice along with placing several seals on the doors or contact points, which helps eliminate wind noises.
As far as putting some kind of hard rope or something in the seal, this can also safely be done as a stop gap measure. When the seals get old, they will collapse. This is due to wear which happens as heat and the door (or what have you) gets opened and closed so many times (think: over many years). When you put the hard rope or cord or what have you in the hollow space, you are making it "puff up" again, thus allowing the seal to do its job for a while longer. As I alluded to, this will work for a while, but sooner or later you're going to have to replace the seal.
To make this work, you need a hard cord. I believe furniture fabric piping might work. It's the stuff which keeps the bead at the fabric joints from collapsing when used. You want solid stuff, not fabric based or anything which will absorb the lubricant. You use something like petroleum jelly or WD-40 to lubricate it before you stuff it in, then push it on through until it comes out the other side. Not an easy task, but can be done. Most weather stripping is designed to easily pull out of its channel (or off of a rail). Then replace it when you're done putting the cord into it. Trying to get any other than a very stiff cord through the hole is like pushing a wet noodle uphill. Very hard to do with very little in the way of results to show for it.
After watching the video, what he's saying is spot on. Would work by doing it that way as well. Maybe even a bit better than what I described above. It works for the same reasons, no doubt. Pulling the filler through versus pushing it would make it a lot easier job.