Self-referential humor is difficult; to be successful, it requires the jokester to possess a level of self-awareness sufficient to recognize and mock their own foibles as perceived by others. When a joke misses the mark, it can come across as an example of the very thing it seems to mock, or worse: as mocking someone other than the joker.
In this case, the joke is aimed at an external audience, one who probably cannot differentiate between SO-the-system and SO-the-social-media-marketing. For a member of such an audience, all that is required is that they are aware that duplicate-closure is a thing on SO.
NOTE: this doesn't make the joke funny. Which may be the bigger crime here...
Folks will overlook a lot of crude and careless behavior for the sake of a really funny joke. Generations of comedians have demonstrated this! But deprived of the LOLz, we're left to analyze the mechanics and accuracy of the joke... And this rarely ends well.
...and, after all, what could be more appropriate for SO than a pile of cringe hacks? 😏