It's a story about squick. The aliens aren't merely having trouble grasping the concept. They are revolted by the concept. Indeed, as you've noticed, their rev

It's a story about squick. The aliens aren't merely having trouble grasping the concept. They are revolted by the concept. Indeed, as you've noticed, their revulsion appears to be the thing which prevents them from getting the picture. It's not as if they haven't seen meat. They've even seen intelligent creatures that are almost entirely meat. That's all part of the joke. If these aliens could get past their squick they'd be clever enough to understand squishy brains, but they can't.