They're totally cannibals. I have two reasons. The show made a point, at least three times I remember to highlight the fact that there is a woman cooking meat. They ended an episode with her creepy ass face asking them if they want a plate. Rick slapped the food out of their hands. There's just too much focus on the food. Also, and this is what sold it for me, Gary referred to them not by their names at the end, but by generic nicknames such as leader, samurai, and archer. If you remember, when they had the pig farm, Rick told Carl not to name the pigs because they were going to just end up as food and that Carl would get attached. Same principle.