To my knowledge, he never disobeyed an order while on active duty, or encouraged anyone else to do so. He expressed his views as an American citizen. Do members of the military forfeit their First Amendment rights while off-duty? If so, something is wrong.
In addition, if the standard in the UCMJ is "undermining the chain of command," then how can his actions "undermine" a chain of command that has already publicly displayed gross dereliction of duty and illegal partisan activity against a duly elected President?