They might have greater dispelling, but they might not have a good enough one to deal with a 17+caster. Perpared for spells is one thing prepared for 9th level spells is entirely another.
Even regular dispel magic would work if cqast by a 10th+ level unless the cleric casting StoV was 20th+ level. I am sure there will be tons of scrolls high level and mid of not only dispel but greater dispel also throughout the army. Not to mention wizards and clerics who most likely have either or both spells prepared also. It's a fantasy world and campaign. Who would have an army without magical support? It's called an army for a reason. To think that one 9th level spell can take out an entire army is silly. Also to think that an army would only take up 720' diameter encampment is kinda silly.