This is a myth. The UCMJ applies only to members of the US Armed forces. It does not apply to anyone else, including combatant prisoners, whether lawful or unlawful.
Also US Civil law does not apply to combatants. That is the problem with all of this. They are trying to apply the wrong laws.
So by what legal standard are the terrorist being tried by in military tribunals if not civil, UCMJ, military or international law?