I'm not in academia so forgive this for being a naive question: is it expected that if you publish a preprint, and someone comes to you with related work that you haven't studied/considered/used, that you then add a citation to that work?
In effect this is erasing independent practitioners as inventors of methods from the history of science, and claiming false credit.
When I think of citations, I think of ways of references that also attribute credit. Maybe this is different in academia? At least for me, I'd see a citation in this specific case as improper, because (assuming they didn't know able the author's work) the paper doesn't include anything from the author of the post. It would seem to me that before you'd include a citation, you'd want to take the time to fully review the other work to see that it is indeed the same idea and also not a load of crock. Depending on the subject, that might be a substantial time investment.