They don’t have to be enemy combatants. Senator Graham specifically asked Kavanaugh about this during the confirmation hearings.
Kavanaugh also mentioned the conditions under which Article I/Section 9 would be implemented, thus opening the way for military tribunals. We aren’t there yet.
There is no category of 'enemy combatant' in the Law Of Armed Conflict (LOAC) under the rules of which these proceedings would occur. The non-State sponsored participants are 'unlawful combatants'. See the Lexicon entry for same.