I suppose I am one of the women they're referring to, although I'd love to know why they say 'full uterus removal' (wtf) rather than just boring old hysterectomy. Had a hysterectomy, can confirm that I don't have periods, am on HRT and have pain, but that's because I've got endometriosis, and these men on reddit are just full of shit, which is presumably what's causing the diarrhoea that they like to pretend is a period.
However, I digress. I think we should stop trying to push back against this with biology and accurate descriptions of the menstrual cycle, and the reason for that is that these men know full well that they aren't having periods. They find pretending that they are having periods sexually exciting. They may well also find talking about it with other people exciting, and the same goes for arguing with women who say no you're not.
However because it's quite difficult for Dave from accounts to rock up to a meeting clutching a heating pad and telling everyone he's got period pain without senior management deciding that it's time to quietly manage him out of his job, men like Dave are acting this pretense out online, where they can anonymously larp with no fear of real world repercussions. Men used to do this in private, now they do it online where we can see them. It's well known that scat play is a thing, as is playing with urine, breastmilk and even vomit. It's not a great leap to understand that for some men, playacting menstruation is the thing that turns them on. This may well be something they only do online - I wouldn't be surprised if quite a few of them present as normal men IRL and their woman persona only exists online.