He still draws a military pension he can't violate the UCMJ. Calling for a coup of the US Government is a violation of his oath. According to the
First Amendment Encyclopedia:
Article 94 of UCMJ gives the parameters for Mutiny and Sedition. Theoretically (I am not a legal scholar or Military expert so it is all in theory) he violates section 2:
More Info That Breaks Down Things a Bit Better Than SUNY-Ithaca
Freedom of Speech does not truly exist in the military and since Flynn is a retired General who still uses rank and gets a pension he is restricted as well. They could recall him to active duty and court martial him. People have been calling for any retired military connected in any way to the Insurrection to be recalled because of this. Just because he is a civilian now doesn't mean he can hint at a violent overthrow of the duly elected government.
It comes down to whether the military has jurisdiction.
The Military Times discusses it here.
(again this is all my opinion based on what I have read on the subject since last November)