Article 88 does not refer to enlisted men/women, active or retired. It does apply to active duty commissioned officers.
Civilians are definitely not covered under the UCMJ. Recalling a retired member of the military to active duty is not an easy task and is covered under regulations that require specific reasons for the recall.
I have never seen a retiree recalled other than when their specialty was badly undermanned or the retiree had a critical skill and had not voluntarily agreed to return to active duty.
Below is Article 88 and commentary from an article about it.
“When a military member is wearing the uniform and receiving a salary from the Department of Defense, that military member has essentially signed away his First Amendment rights granted by the Constitution.
The exact words of the Uniform Code of Military Justice Article 88 - Contempt Toward Public Officials states: “Any commissioned officer who uses contemptuous words against the President, the Vice President, Congress, the Secretary of Defense, the Secretary of a military department, the Secretary of Transportation, or the Governor or legislature of any State, Territory, Commonwealth, or possession in which he is on duty or present shall be punished as a court-martial may direct.”
The main reason for this regulation is to keep military members who have access to major weapons of war to ever get involved in politics. Once they are retired or resigned their commission and a civilian citizen, they may partake in such political arguments in both written or spoken word. With the advent of social media it can be a slippery slope for military members to discuss such matters and could even be subject to UCMJ violations. That is why you will find military members refrain from that activity or have incognito social media accounts.
What Determines Contempt Toward Officials
(1) That the accused was a commissioned officer of the United States armed forces;
(2) That the accused used certain words against an official or legislature named in the article;
(3) That by an act of the accused these words came to the knowledge of a person other than the accused; and
(4) That the words used were contemptuous, either in themselves or by virtue of the circumstances under which they were used. Note: If the words were against a Governor or legislature, add the following element
(5) That the accused was then present in the State, Territory, Commonwealth, or possession of the Governor or legislature concerned.”
That is kinda what I said, in one sentence.