yes, you're right, the idea of including women in a group of genetically enhanced soldiers who are augmented and mutated to the point where they're no longer considered human and already had a new iteration become canon in a universe where "canon" is a vague notion that can be changed on a whim would be lore breaking and just feel like forced diversity
like, seriously, I want you to think of what gaming was like when 40k was new and they came up with the idea that only men can become space marines. I want you to think of why they made that statement and why now they might do away with that notion. like sheesh, they haven't even said anything of that sort to my knowledge, but the fact that this is a sacred cow of the highest order for some fans certainly reveals priorities.
Or you could look at it positively. Horus Heresy novel 28 - Scars:
"Malcador smiled. 'You brothers - such a nest of rivalries. I warned him to make you sisters, that it would make things more civilised. He thought I was joking. I wasn't.' "
If there had been female primarchs and marines, perhaps there would never have been a heresy.