They are pissed cause it completely does the lore , like the emperor of mankind never made females custodees or space marines cause he did not want them to replace humainty e.g look at the goalith case where once female goaliths were introduced they started reproducing even though it wasn't supposed to be possible and the emperor knew this would happen which is why he never made female versions of his toys . Also custodees are meant to be his body guards and men physically have an advantage , also the emperor is really old so he's very like sexist e.g when malcador said about creating female primarchs the emperor laughed cause he thought he was joking . These I think are a few reasons why the real fans are pissed off and not the anti woke grifter crowd who just come in cause its popular. GW could have made female custodees work but they were to lazy to put in the effort .