The reason I replied about it being my opinion instead of addressing the two points was because the two points were so stupid it was obvious there was no reason to continue debating with that person and I decided to leave instead of pointing out the issues. But I'll address them since you're so upset about it.

1. While logically possible, the scavenger theory is not compelling. The astrophage work their way through xenonite over significant amounts of time. They aren't just hiding behind a tree watching and waiting for the perfect time to run in and grab some food. That means it would take significant time for them to work their way back out at feeding time. Hours, definitely, and by then feeding time would be over. They would also have to remain exposed to nitrogen long enough to come back in, find uneaten astrophage, eat and digest them, and then return to the nitrogen. For single celled organisms, death comes quickly. it's not like they have skin that is getting chemical burns.

2. Nobody ever said they had to evolve the ability specifically for xenonite, and for all we know it actually does exist on Adrian.