If the negative externalities are the same in both cases, then this isn't an excuse. Just because Facebook has proportionally fewer posts that violate its policies doesn't mean it should get a free pass. The fact that up to 15,000 people have PTSD is the societal cost we pay, even if the vast majority of FB users are using the platform as expected.

Facebook gets a free pass because Facebook is an influential organization. Parler had no network of elites protecting it, because as you say, it had no other purpose beyond being 'volunteer' moderated.

If at a minimum, the attack gets us to think about the type of questions posed by The Social Dilemma, we're trending towards a better place.