IMO that's more appropriate than UCMJ. Rioting is a civilian thing. But the mil could still get around to him... civil charges do not preclude double jeopardy, that is N/A for civil + UCMJ.

If he actually attempted to organize current or retired mil members to an insurrection, then I could see UCMJ being relevant. In that case he's acting in a military capacity. By "insurrection" I mean organized application against the gov of military/paramilitary combat power by movement, maneuver, fire, etc. As opposed to a riot... what I saw in DC looked like a riot. CHAZ for example was closer to insurrection, they seized and held sovereign US territory by force of arms.