Yes, I understand that 4! is the correct answer. That is not my point. Without the misconception that the answer is 4, there is no punchline. This comic strongly implies that the natural assumption should be that the answer is 4, and that the secret, hidden answer is 24. In other words, in a world where people understand order of operations, the comic is not funny, because you wouldn't look at that and think 4 is a reasonable answer.
The joke relies on, as you put it, the assumption you did it wrong.