There are a lot of 10/10's out there for this book. Also a lot of indifference. Which is fucked up if you ask me. 10/10.
Well my score for this book was 9.5/10, which while higher than yours is not a 10/10 but admittedly close to it. But I felt that AE fell short of that 10/10 rank due to some unexplained, or not explained enough, events like the DE ship and one or two things that I found confusing.
But I enjoyed seeing so many familiar faces return, it's a nice reminder that these characters were around then and seeing how they became the characters that we know really made this book better imo rather than just introducing a new cast from scratch just for the purpose of this one novel. I get why some wouldn't like what McNeill did and i'm not saying you should, i'm just saying that I did and thus I enjoyed the book more and gave it a higher score.
As for your mention of the Iron Warriors being backstabbers, I think that comes from the position of the Triarchs and the idea that a leader can be demoted for failure, even if it's not a bad one, and that would naturally encourage others to try and gain an advantage over others and of course that naturally leads to backstabbing, especially if bitterness becomes a problem in the Legion as we know it did with the IVth, and if your commanding officer doesn't put a stop to it as Perturabo clearly didn't.