The i is a leftover from the 1970s, when a few companies (BMW and Mercedes, notably) started replacing the carburetor on their petrol engines with fuel injection. The marketing department saw an opportunity (for bragging about the superior fuel management on those cars) and added the i (or E, in Mercedes' case, for Einspritzung, German for injection) to the car's type number.
This worked for a few years, then the catalytic converter became mandatory and everyone switched to fuel injection. 45 years later, BMW still uses the i (as in '320i'). To make things more confusing, they are labelling their electric cars as i3 and i8.
Mercedes dug themselves a hole with their old naming scheme (which became increasingly convoluted with the introduction of the 190), so they switched to using letters (C, E, S etc.) indicating the model range. The 300 E became the E 300. So their midrange product is now called the E-class, and you can have an E-class diesel.
Similar things happened with other technologies. Turbocompressors had a T in the type name (esp. turbodiesels, TD), then came intercoolers (TDic or TDI), direct fuel injection (GDI, *SI). Most of these disappear when they become ubiquitous, some endure.
The list itself is a bit misleading. Take this entry for example, for Honda:
Petrol: i-VTEC
Diesel: i-DTEC
Hybrid: IMA Hybrid
Honda doesn't call all its petrol engines i-VTEC, just the engines that use the i-VTEC variable valve timing system. The same goes for other entries in the list: lots of them are specific technologies (or engine series), not generic petrol/diesel indicators.
