"a person who uses unlawful violence and intimidation, especially against civilians, in the pursuit of political aims" is a terrorist, ie the IRA or ISIS. A loner who hates women is not necessarily promoting a political agenda beyond his anger, but a white supremacist could be.
What proof do we have of that as yet? One facebook post that may not have been his. He did act alone and when I follow the incel crumbs they lead to a woman in toronto who started the group. It is sad that so many people are alone and hurting and feel such a group is helpful