White supremacist without further elaboration is a better story for the police and prosecutors than police provocateur (and quite possibly not technically false, either as to affiliation or motivation for the action, even if incomplete because the latter is also true), so if it's actually the latter, the former is a more convenient narrative but not one which would likely be able to be maintained if there was a trial.

I think the plan is to not charge most of the rioters. If they charge him, they have to charge all the others too.