If they were able to board then there must have been stairs available (either the integral ones or mobile ones) so why not ask the dispatcher to get the stairs re-attached and open the door(s)?
It would still have been very hot - but would have helped a bit (especially if door open at both ends and can start a breeze going through).
I've been on a 747 before which was stuck on the ground in South Africa with air con turned to max and it was still pretty uncomfortable with apologies being broadcast over the PA system.
I suspect that they didn't provide water either (I know there is a very limited supply of bottled water on board and I understand they have been told they aren't allowed to give it out for free, or sell it when on the ground).
Glad I wasn't on that plane!