No because 4 is the obvious wrong answer. 21+3 my isses idea completely.
4(exclamation) is wrong while 4(factorial) is right.
I think the smugness of someone who knows factorials is also part of what makes it fun.
No the joke is that first think they got wrong, then realize it's a factorial so they got it right (the a-ha moment!).
Then we realize that they might not have gotten it because the wrong answer is 4 and they may not know of factorials.
The ambiguity is very clever. Some other commenters call it a pun.
Soon they may come with cellular capacity. Cars and e-bikes already do.
You gotta Faraday cage it!