Wasn't Chaos actively empowering them to try and screw the Imperium at the time? And the answer would be "Elite" vs "Mook" always ignores which is which. Just look at Catachan Devil units. Also, you're argument is flawed. It's "There is this thing that disagrees with your point, therefore you're wrong even though there is also that thing which proves your point." There is evidence of both. However, a Custodes being ganged up on by Astartes is quite different from a squad of Custodes working together to fighting through groups of Astartes. Besides, it's likely that the Custodes on Prospero didn't face large numbers of Astartes at a time and when outnumbered teamwork would win them each skirmish regardless.





Wasn't there that guy who punched through a Custodes' armor and ripped out his spine or something? Still, Chaos would explain all of this seeing as these guys were the main threat to Chaos's plans at the time due to getting in the way.

