that commenter was me. Try ctrl+f.
Your explanation of the joke would suggest that it would be equally funny if the question were "what's 21 + 3?" But obviously that isn't true -- the joke is meant to be that both 4 and 4! are equally valid answers to the question. However, in reality, 4 is not a valid answer; I just feel the same disappointment any prescriptivist feels when they see a word misused.