I can take a guess. After the Buddha was enlightened, he went back to his old friends the ascetics, and they didn't believe he was enlightened. But they talked

I can take a guess. After the Buddha was enlightened, he went back to his old friends the ascetics, and they didn't believe he was enlightened. But they talked and after a while they were convinced. Later, doubters would visit the Buddha and they'd have something like debates in front of the other monks. So I think Horn is saying he wants to show through debate that Massaro's views are bad, i.e., not the dhamma (truth).