Except ANTIFA was never declared a terrorist organization. (The President threatened to, but it never happened, unfortunately.)
But in the middle of protest season, I saw an unmarked car arrive, three soldier type guys get out and pick up a woman from the protesters - zoned in on her immediately. Never spoke - just put her in an unmarked car while her friends demanded they identify themselves. Their uniforms were masked to obscure who they were with.
Posse Comitatus is generally not thought to be binding on the marines - of which President Trump is the Commander in Chief. I think we have crossed a legal threshold and the reason we aren't seeing more "fallout" over it is that Trump is limiting his own conduct. Not sure about the antifa designation, not sure if it's unannounced or on the griddle. There's a senate resolution here:
https://www.congress.gov/bill/116th-congress/senate-resolution/279