they went with the bioengineering aspect of the Alien as seen in the deleted scene in
Alien Director's Cut where Ripley finds Dallas, half-alive in the process of dissolving and turning into a cocoon who begs Ripley to kill him, and a dead
Lambert Brett in the process of turning into an egg ("eggmorphing"). So I thought that Creative Assembly went with that explanation.
Thoughts: Honestly, the first time I saw that scene in
Alien Director's Cut, I found it stupid and annoying merely out of habit and comfort; I had grown up with the idea of a Queen from Aliens. In time, however, I've come to appreciate that capability of the Alien, a self-sustaining killer and natural bioengineer, and the sheer horror of turning people, dead or alive, into cocoon and eggs while I came to find the very idea of a Queen sort of insulting, reducing the magnificent biomechanical killing machine that is Alien whose "structural perfection is matched only by its hostility" as Ash says, down to mere bugs with a buglike reproductive cycle and group behaviour. So I was very glad to see a reproductive cycle without a Queen in the game.
I still find that that deleted scene breaks the film's pace, though.
Read more about "eggmorphing" here with notes and anecdotes from people involved with the film:
https://alienseries.wordpress.com/2013/11/23/egg-morph/