If Alien Earth is canon then it will not result in an infestation. If it is not canon, then it could be indeed an infestation. Potentially confusing, yes. But perhaps also an interesting test to see if there is any interest in that sort of story, though execution is everything. If we have a Bladerunner-type world with bugs in it, that would be epic.
Plagiarus Praepotens really dials up the stakes on an earth infestation though. If it were just traditional aliens of old, those could be wiped with some effort, but if literally every animal could be contaminated with black goo, I don't see a way to fix that, short of Engineers just snapping Thakor fingers and saying, let it stop.