Another user in this post’s comments explains that viewpoint fairly well I believe, here’s what they said:

They want to remind these men that they’re different than most men, and because they’re different they should feel lesser, or that they shouldn’t even consider themselves a man. It’s a transphobic dogwhistle; it has plausible deniability as “discussion” or “fact” but carries a strong transphobic connotation.

Same with why users are arguing so passionately that “men” has to always mean “cis men” and never include “trans men” because “most men men are cis.” These are the same people who in the 1980’s would have said “men” has to always mean “straight men” and not “gay men” because “most men are straight, it’s just a fact.” Or back in the 1780’s would have said “men” has to always mean “white men” and not “black men” because “most men are white men, sorry I’m just stating facts.”

The average person, of any identity, is not gonna care if men can get pregnant or not. Most people who were AFAB don’t even want to get pregnant or be associated with it, and someone who is advocating for trans issues wouldn’t consider this topic to be pressing matter.

So when someone takes the time to leave “men can’t get pregnant” in a comment section of a post, they likely aren’t doing so because they’re passionate about “civil discussion,” or “biology.” Especially when they’re doing so in a sub dedicated to the belittlement of others.