It's not clear if you want to ignore the entire thematic and narrative implications of the script, I guess.
David 100% made the Alien as we know it. Now, the Pathogen that he used as the core base substance absolutely comes from unknown origin (and the Pathogen has been known to create Alien-like beings in the past without David's meddling; the mural and the Deacon/Neomorph, the acid blood in the Hammerpede, the gestation method of the Trilobite, etc. are all testaments to that) so there is definitely still an air of mystery as to where the raw materials that David worked with came from. But the capital-A-Alien? All David's, due to his refining of the Pathogen and his experiments in combining it with other ideas and substances that are very much in line with his, ehm, interests.