The implication was that there was still food left. Kirk mentioned, during his speech on what is to come for them, that soon there will be no more food, maybe 6 months worth still left.
KIRK: And the little ones. What's going to happen to them after you've gone, after you've turned into creatures like Louise? Oh, they'll still be here, but not for long, because the food's all gone. You've eaten it. Maybe six months left, that's all, and then nothing left to eat, nobody left to take care of them. They'll die, too.
Perhaps the Grups, knowing their time was finite, prepared an overstock of preserved, canned food for the onlies.