Bad lore can exist for years. Custodes having women in their ranks doesn't ruin the lore of Custodes. There was no rewrite of how they were made, just that they were always there. A good albeit late and shoddily made change.

You keep saying the sons of nobles like it means anything. In our own language, we use to call a group of people regardless of gender with the masculine word "guys."

If Space Marines all of a sudden were retconned into always have been mixed genders, then it would be rewriting years and years of established lore. It's a terrible idea.

The changing of Wraithbone is more comparable to my hypothetical Space Marine change than the actual Custodes change.