If you're not the ultramarines you get nothing because GW whenever stop giving all the fun stuff to gillman.
Also the alpha legions entire actions make absolutely no logical sense if the entire legion or experts in counter information warfare psychological warfare an information gathering they would know the entire time the cabal was full of it and every single thing they were being told was a complete and utter fabrication.
Really the whole cabal subplot was really really stupid it was frankly completely and utterly abandoned leader by GW in any future editions because they realize how stupid it was.
The alpha legions for experts in completely ripping people apart psychologically militarily and with special operation units they also use infiltration tactics the primarchs guys in the dumbest way ever I'm just gonna head Canon that away is the alpha legion never took part in the horse heresy and are actually just chilling doing something very important for the emperor.
Top secret no one's allowed to know about it hey that would be fun thing to do for GW could also introduce a bunch of new factions maybe the only super far away edge of the Milky Way Galaxy and you discover new aliens you could sell so many new miniatures