The point was that a passenger isn't allowed to buy a hamburger for a private pilot as "compensation", even though it only costs ~$10 and doesn't even remotely cover expenses. (In the case of a "$100 hamburger run", going to a restaurant at a distant airport may be the intended destination -- so offering to buy someone's meal at said restaurant might otherwise assumed to be a reasonable thing to do.)

You are, however, allowed to split expenses evenly with passengers -- provided everyone is sharing the same common purpose (e.g. flying for fun). But the key part here is that the pilot isn't allowed to receive anything that could be remotely interpreted as compensation.