All we get from David in terms of being incorrect about something- is authorship, not details- otherwise he couldn't (re)create the Alien in the first place. The Pathogen, and therefore the Alien created from it- is a E.T, A.I- the RPG itself reiterates this.
You seem to fail to grasp that David is a liar. He's actively deceiving the humans, and to an extent, also deceiving Walter. The reveal that he killed and replaced Walter puts everything he said, including his role as creator of the XX121, into doubt. He might think he is, and not be (a delusion). It might even have all been a deception.
The only thing he says in prometheus we can trust as a certainty is that he thinks
he perfected the alien agent into xx121. And maybe his rationale for why he's doing it. He's dangerously unreliable. We see him arrange to disburse the XX121 aliens