I don’t think this is their argument at all.
Facebook regulates and removes far more content than required by law.
Under the US 1st Amendment almost everything offensive, racist, bad, pornographic, etc is legal. Facebook still removes quite a lot of legal content from their posts and their content moderators get PTSD from having to look at content which may be legal.
I think Facebook’s real issue is that they know they and their user base want them to be stricter than the law requires, but where should those standards fall?