Nothing in the argument is taken to prove a hypothesis that penicillin use can lead to penicillin-resistant bacteria. It’s also not a hypothesis at all, it’s an assumption that’s taken for granted and treated as a fact.

The scientists do not believe that penicillin leading to resistance would mean that penicillin is the only way to create resistance. The entire argument is based on the belief that some other factor could lead to resistance, and the scientists just have to figure out what.

This is exactly what the assumption about penicillin causing resistance does in the argument. It’s the reason that resistance in the absence of penicillin is confusing, leading to the scientists doing this research in the first place.

Nothing in this argument falsifies anything else. Mercury causing penicillin resistance doesn’t mean that penicillin can’t also cause penicillin resistance. Both are treated as compatible.

This is a trap; complicated does not mean correct. The assumption that penicillin can cause resistance doesn’t prove anything. It’s context for why the scientists are confused to start with.