From what I can gather Arch is old guard 40k, been around for decades. But so am I, so I can firmly tell him to go fuck himself he has no understanding of 40k lore.
Even back when the lore was women couldn't be space marines because certain geneseed required the male adolescents body to fully develop, something about needing a certain amount of testosterone. But that's A. bad sci fi, and B. Old lore and its become an enshrined 'No girls allowed' sign that most of us just sigh at as a relic of 80s British culture.
Nowadays, and Im guessing Arch hates it, but there's a new breed of Marine and they've done a sorta soft reboot of a lot of the genetic flaws/hiccups Marines had, because it was getting a little out of control with dragon humans and werewolves and goat marines, etc...
So now everyone is back to square 1 not a furry Marine, and there have been hints that the next step for Marines will be fixing it so women can become Marines, both in lore they've hinted and GW openly says that if they think it'll make money they'll do it. And there is momentum building because players like Arch, old guard who are too conservative/sexist/racists to allow sci fi fake women 40,000 years from now to be genetically engineered werewolf space vikings because it diminishes their IRL masculinity, are basically being uninvited from the hobby. And there has been a serious outreach to women and girls recently to try and get them into the hobby, and if enough little girls want toy models of women as space marines GW ain't about to turn down the sales, even if they have to Star Trek their way to it lore wise.
Point being Arch doesn't seem to understand even the out of date sexist reasons why there are no female space marines and instead used his IRL sexist disregard for women as the reason.