I've seen this argument on so many threads already, running around and around and around. The problem is that Covenant heavily implies that David is the original creator, whilst also providing just enough wiggle room that it might not be the truth.
The additional media also clouds the issue, with ADF implying it's a recreation, and Advent also implying the same.
Ridley Scott's intent cannot be trusted because he changes his mind so often that what is intended one day could be completely wrong the next.
Basically, there's no definitive answer, right now. I'm stressing definitive, because it's the most important word. We know that David created the alien in Covenant. We know the implications are that David is the original creator of the species itself, regardless of our personal tastes.
The reason this argument goes on and on is that there is just enough doubt that until the movies themselves outright prove that David is the original creator, then maybe he isn't.
Because it's a beloved franchise, and the concept of the alien species being created by an android is disliked by people, a lot of the arguing is going to be because people just don't want it to be the case. You can say they are grasping at straws, but while there's enough doubt in the storyline for the straws to be there, people will grasp at them.