How do they figure the detainees are covered under the Geneva convention?
The question of whether a combatant is legal or not is a determination that Geneva anticipates will be made by a "competent tribunal". It appears (I obviously haven't read it yet) that the SC is here saying that these tribunals don't qualify under Geneva.
So it isn't "the detainees" that are being ruled on as outside of Geneva's rules, but the tribunals' form.