I would say maybe they created it before.
But who knows, i think the Engineers or at least the Elders are very very ancient they have created Life over and over, we cant even imagine how Advanced they are, but we have to Question, as Prometheus showed us... surely as they are just Mortals who possess Advanced Technology, then did their Original Ancestors have to Evolve to create such things?
Or did they too obtain their Technology from a more Advanced Alien Race...
So looking how old these beings are, then maybe Millions of years ago they have had their own Ups and Downs as far as various Creations... maybe they have created AI in the past and barely survived the consequences of this.
We do see these beings think they are superior and have no care for Humans, we do them would be no more than a Basic Monkey, or Guinea Pig.
They would not have much of a Moral Compass, not as far as anything effecting themselves.... just as Mankind Tests on Animals, we dont view them as Humans.
so if we look at David 8 as being a Synthetic Being, well then we also look at the Replicants in Blade Runner, who are Flesh and Blood, surely they would be seen as more Human and treated as equal.. we can understand David is a Synthetic and not real.
But to Clone Humanoids and Tamper with their Memories/Brain and Genetics to limit their Life Span, is playing God its a bit more Immoral than with Androids.
Not if you view yourself as a Superior being....
And so indeed the Engineers in effect could create Organic Life instead of AI, maybe the Engineers are like this they serve their masters and are like David or more like Replicants.
If these Engineer Replicants then Rebelled and then became Free and Sentient and Created their own Creations, they would have the Fear that their own Creations would one day Rebel against them.