My personal pet theory on this, having read everyone's conversation, is that since Loki is translating it... whatever the actual word is, it's not ACTUALLY Alpha, it's just the most fitting name he could think of.

Progenitor means "creator" means "has created things, existed before those things", and in this universe it's a well known thing that Omega is a term ascribed to the end of all things, so the counterpart to the end of all things is the beginning of all things, so Alpha. I can see him going down that chain of thought. It's just not a wholly accurate translation. it's the Progenitor, roughly, in the Celestial language, because it is capable of progenitoring things, and Loki read that as a more direct Creator then just "someone who creates".