He's just a troll is a silencing tactic and excuse for sexist incidents in which the perpetrator is explained as a troll, that is, someone who doesn't really hold sexist viewpoints but expresses them in order to annoy people, upset people, or play and experiment with social norms. There are several problems with this: It's still intended to harm women in any case. A troll may not believe himself when he says that, eg, women are provably less intelligent, but often the claim is intended to upset or undermine a particular woman.