Not true. It's each battle, and eventually your materia can level up to give you up to 5 summons from the same materia. Maybe their caster equipped with Ghost of Kiev died and no one has a Phoenix down, revive materia (probably equipped to that unlucky soul), and they probably didn't do Fort Condor yet. Womp womp
Probably the most likely. Probably relied on aerith/aeris for extra healing too. Or... Maybe they had her equipped with G.O.K. before y'know, the thing.