The absolute truth which is implied when you talk about "canonical background" will never be known because of this.
How does that, in any way, suggest that there are absolutes when it comes to 40k canon? It's not my argument. That comes from one of the BL authors, a guy who has worked on a lot of 40k stuff.
Nothing is fixed? That's the whole point! GW could turn around and say the Emperor was actually an alien with the next edition/book/game. They've done crazy stuff like that before. Roboute Guilliman went from being an Imperial Guard commander, a puppet of the High Lords of Terra, to a Primarch and founder of the Ultramarines. The setting went from fantasy-in-space to the grimdark sci fi we have now. Tomorrow there could be an official female SM Chapter. Or maybe the Squats will rise up and wipe out the Tyranids. Maybe the story will be that the Tyranids are actually only the forward forces of a much bigger, deadlier alien menace that will finally bring the Eldar, Tau, and humanity together. Perhaps the entire setting is just a dream in the minds-eye of the dying Emperor and the universe has actually been scoured free of life by the Tyranids.
Is any of that likely? Hell no, but the very message GW is trying to get across is that anything could happen. Anything could be true. Or not be true. Is it just a reason to excuse constant "canon" changes and the contradictions that come from having so many different writers producing material. Maybe. Probably. But GW and BL are the ones putting out this material, so they get to make the rules.
Understand, I'm not saying female Space Marines are canon or even should be. I'm just trying to say...don't rain on someone else's parade for wanting to make the game more fun. Don't bring canon into the discussion when there is no rock solid, ironclad canon to stand on. Everyone just have fun in the cool, FICTIONAL sandbox that we've been given.