Rats always tend to hide some of the tastiest food for later if there is a bit more of it (you always have to look for and remove potentially rotting food with them), but won't necessarily watch it, and then another rat may eat it if they find the place. Only obviously in a small plastic box there are no good hiding places to be had, so it may just drop it in a corner or the like. In such surroundings you just can't tell how it would normally act. So I'm not convinced that the rat necessarily intended to save it for the other rat, unless the experimental setup offered a way to distinguish general food storing behavior from sharing. Also not all rats actually like chocolate. I had some that didn't like it at all or just nibbled. Though if it only has pellets otherwise and already ate the larger portion the rat probably liked it well enough.

Unfortunately these awful, small plastic boxes are how lab rats are usually kept afaik, only that often they won't just put in two in a box of that size but even more. There are actually a growing number of researchers who realize that the way rats and mice are kept for things like drug research and illness models distorts all kinds of results even if you accept premises like the "mouse model" being like humans for their research purposes, simply because they don't research on healthy animals, but on stressed animals that can't move around and don't have anything to do but eat to combat boredom.