PT118 · S1 · Q2 Commentator: Many people argue that

We don't care how frequently supernovae damage the ozone layer. The author isn't claiming they occur a lot. She's just claiming that if THAT didn't do ultraviolet related harm, then how could today's chlorofluorocarbon problem do harm.

The "continue to" damage the ozone layer today indicates that this is not necessarily anything different between the two situations. If we had said, "300,000 years ago, there was very little volcanism, whereas nowadays volcanism has depleted the ozone layer to a very vulnerable state", then THAT would be pointing out a meaningful difference that would allow us to say that the less-impactful effect of chlorofluorocarbon (compared to the supernova) could still lead to harm.

This would be true in both situations, and this doesn't give us a way to argue that modern chlorofluorocarbon damage, which is less severe than ancient supernova damage, is more likely to harm humans. It's possible someone was thinking, "Maybe the supernova's damage and the chlorofluorocarbon's damage is cumulative. i.e., the punches landed in Round 1 were harder than the punches landed in Round 10, but because the boxer was so weakened by Round 10, a weaker punch could still knock them out." If so, this answer would actually work against that type of objection, by saying that the ozone layer regenerates its strength, so whatever damage the supernova did 300,000 years ago would probably have been erased by now, as the ozone layer slowly regenerated.

This is similar to our correct answer, except this answer gives us no specific sense in which humans genetically changed. Did they change in a way that made them more / less resistant to ultraviolet radiation? Did they change genetically in some way that suggests that the ultraviolet radiation following the supernova harmed them? We don't know and this doesn't say, so it has no clear impact.