There is also the fact that the AL primarchs were shorter than the other primarchs and could be seen as just a tall marine.
I'm holding on to this theory because it just makes sense with the Alpha Legion strategy of often hiding in plain sight. The true warlord being a random character, deceptions to trick the enemy into attacking their allies, a legionary disguising himself as an enemy soldier. And ultimately, the reason why Alpharius keeps reappearing even though he has constantly been killed; because every single Alpha Legionary can be Alpharius. You cut the head off of the hydra, and two more appear.