According to the lore (and novel "Legion" by Dan Abnett), Alpharius alone amongst Primarchs wasnt a 4-meters tall god-like giant - he was of the same height (about 7,5 feet) as his fellow warriors - that's why he could easily change his role with any legionnaire
Yet we then get to the awkward point where once again the kind of history knowledge a Space Marine would have they would know that Alpharius was basically completely destroyed by Rogal Dorn. He is 100% confirmed dead, no Alpha Legion Bull. So loyalist Space Marines, especially when the two present are one who has a near dangerous liking pretty much any knowledge and a Crimson Fist, would likely laugh at the mere implication. Though obviously considering how restricted such history is in the Imperium not many outside the Space Marines and the main Ordos of the Inquisition would really know.
The Heresy and the actions within it was nearly 10,000 years ago from where we are. Much of its records are lost and even with the Astartes there is a lot of myth and legend. In a Legion that specializes in pretending to be their Primarch, Santana might dismiss the claim that it is Alpharius, but he'd still be wary of if it was actually true.
Do they know indeed? Hmmm....well, at least Rackham doesnt . Alright, I'll reveal his true code name to loyalist space marines. Dont you know, "we are all Alpharius!"
And Alpha Legionaires are a little bit taller than other space marines. Regarding the Alpharius thing. It is a tradition that should be upheld. Like using "For the Emperor!" as battlecry. Most persons know the truth, but it has the potential to confuse enemies. And if Alpharius was still alive, the legion would not tell anyone by using his name. Or would they?
I can picture that being a last moment plot twist and you just show them flying off in their ship like View: https://m.youtube.com/watch?v=lt4329_W_s0
I already planned that scene, if Sabios happens to be Alpharius, but I cannot tell you whether this is true or not and what Sabios knows about this.
