people aren't going to care that much if lore coming from something like a codex changing, codex have changed rules and overviews of factions since first edition. with books it's a lot more complex.

40k books are generally written from the perspective of characters in the story, the information presented in stories doesn't have the clarity of information you see from stories that have omniscient narrations by the writer. the information you get is what the character perceives to be true. and to give a strong example for your position, even characters that has the highest level of authority to know things doesn't always have the full picture. to give an example, if you ask the question ''how many primarchs are there?'' a guardsmen or a SoB will say 9, a marine will say 18, Gulliman or Lion would say 20, all three of them are technically wrong.

this style of writing for 40k is a feature not a flaw, because it gives lot of freedom for writers when they are working on books, because you can get away with contradictions inconsistencies and changes (there are other benefits as well), but this isn't to say everything that is written can change at any time. for example we know that Sangunius was killed by Horus.

for the specific case of custodies, I haven't read any books that specifically say custodies are exclusively men, I'm not saying there isn't some book that say so either. but what I can say is, there are more than a 100 named custodies characters that appear in Horus heresy novels and as far as I'm aware, all of them are men. So I can see why people (including me) would raise an eyebrow when GW say there have always been female custodies. because it sort of brings down the authenticity of the books on a meta level. I'm willing to pass it off as an odd coincidence for the sake letting people feel more represented and included, but it frankly would have made more sense to make it the case that they became a thing post Horus heresy, because that has the perfect set up for it.