There's an interesting bit of complexity here though because a fair amount of Mann's status as a public figure is because of the climate-conspiracy community's singling him and his work out as a target to be attacked.
If a group elevates someone to be a public figure, and uses the fact that they're a public figure to defame them in ways that would not be permitted for non-public figures, should the group really be allowed the same protections that random members of the public have talking crap about people who are incidentally famous?
(I suppose it gets a bit complicated that Mann didn't retreat from the attacks, and used the notoriety thrust upon him to become a fairly prominent science communicator. Even so, giving his attackers any benefit of the doubt doesn't seem right somehow...)
There's some precise way to say the same thing without falling afoul of it, even then.