Hey @voronoi, good questions. I didn't read who the poster was for a bit and was like "hmmm who this critical thinker?"
I should have done a better job making clear that I think the meat-for-sex hypothesis is pretty well rebuked - I don't buy it. Maybe it happens at this one site, but see my response to @oldfangle for more info on who actually gets the meat. An extra detail: anecdotally at Ngogo, old individuals seem to commonly get meat, males and females alike. What's up with that??
Colobus are largely folivorous, leaf-eaters, but will eat fruit as well. I think that even if their diet overlapped more, this would not explain their targeting as prey. Feeding competition is rarely, I dont think ever, a explanatory reason for predation
Let's consider the evolutionary math, it's all about costs and benefits for the individual. Sure, the chimpanzee group hypothetically has more food if there are no colobus in their territory, but what is the return for a single chimpanzee hunting? Those benefits would be far in the future with pop. decline, displaced benefits like this are much weaker evolutionarily speaking. Also, benefits would shared across the entire group, but hunting is personally costly and dangerous. If a chimpanzee is worried about food security, that time is better spent just eating more, perhaps storing calories in fat, rather than running off to hunt for a possible-much-later food benefit. Maybe group benefits do matter, but individual aspects are more salient.
So why catch some prey over others? Consider the chance of finding prey, the cost of obtaining the prey (energetically and in terms of danger), and then the nutritional value of the carcass. Duiker are solitary and fast on the ground. Colobus live in groups and can be cornered at the top of a tree. This might also explain why they hunt red colobus more than black and white, perhaps the later are just better at getting away.