This seems an overly complicated way to reach this conclusion. Should lunar microbes have existed, it's unlikely they would have preferentially targeted humans,

This seems an overly complicated way to reach this conclusion. Should lunar microbes have existed, it's unlikely they would have preferentially targeted humans, or land animals, or macroscopic animals, or animals, or eukaryotes, etc. "Quarantine" was broken at the moment the hatch was opened, if not before.