Additionally, it seems likes there's really no way to interact with this approach that isn't "yep, hilarious, this is canon to me now" or pedantic nitpicking. I don't even add the pedantic nitpicking in most of these, but some fandoms are more prone to it, and when the OP of this kind of post acknowledges the responses at all, it'll generally be with some tongue-in-cheek "explanation" of why their headcanon actually does work that makes no more sense in terms of the story, but which is presumably funny, and forceful enough to be convincing if you're not particularly into the fandom. Some of them, in fairness, will admit that they simply forgot or don't know the story that well and just thought it'd be funny, or "I choose to reject X because my headcanon is funnier, but I know it's there." But more often, I see half-mocking "actually I'm right [but you're a humorless asshole if you actually engage in any way other than agreement]" defenses.
Of course, nobody appointed me god empress of fandom or anything. There's nothing wrong per se with people making posts without being obsessively into the original material or enjoying fandom in a way I don't. But that form of defensive humor really does not work for me. And there's something about an approach to fandom that's dominated by snide, viral BNF humor that doesn't need to make any sense in terms of what it's ostensibly about, that doesn't even need its audience or author to know what it's about beyond the vaguest pop culture osmosis/online memes and is better if they don't, that I find both obnoxious and just kind of ... sad, I guess? I didn't come into fandom in the truly early days, but it was early enough that everyone I encountered had intense feelings about some aspect of the thing they personally had read or seen or heard. Even people with incredibly bad takes seemed to generally be an emotionally invested fan or hater of the actual story.