Eh, David being the sole creator of all xenos is pretty easily retcon-able anyway.
"He was wrong, and it was just the one strain."
That could even get you a situation where the black goo will, sooner or later, create something Xenomorph-ish, which could imply the elixir of life has an "undo" button - a pretty cool concept that elevates the Xenos back to a more Lovecraftian threat.
If that sounds dumb, remember: It's this or time travel.