Calling it a basic genjutsu is laughable given there's no justification for that statement, but Hiruzen couldn't dispel it(assuming that's even possible with that genjutsu) because he'd already begun Shiki Fuujin after creating two shadow clones. So his chakra was spent, his body was immobilized with summoning the death god, and then after the death god had been summoned, he was on a time limit so it seems rather than waste the time to dispel(assuming that's possible with that genjutsu), he just used his sense of smell and captured both edo kages(since that's something he had to accomplish regardless). Claiming he couldn't dispel it is saying two things: It's possible, which we don't know for sure. And two that he wanted to anyway, given he was on short time and had other methods of stopping the technique.
Being caught by the sharingan genjutsu requires staring an uchiha in the eyes, which if you're going to try and argue Hiruzen is stupid enough to do, there's no reason for us to continue our conversation.