Not "...unless they lower the price". He made a prediction, and in hindsight, it turned out to be idiotic. Maybe he was assuming that the price wouldn't drop (a monumentally stupid assumption if that was the case).
He made his "prediction" based on what was known about the iPhone at the time. At the time, it was reasonable. A lot of people were saying that same thing. The iPhone was too expensive. The real problem is that a lot of Mac fans didn't want to hear someone like him pointing out such an obvious negative about the iPhone.
As far as accusing Ballmer of stupidity for an assumption that the price wouldn't drop, well, you may as well accuse everyone of stupidity for not predicting that Apple would drop the price so soon. That was a strange and inexplicable move on Apple's part and of course nobody saw that coming, including Ballmer. Apple hadn't sold enough iPhones by that point to justify a price drop so how do you explain it? If Apple had sold the iPhone for a year at the original price and had great success, then Ballmer would have been flat-out wrong. But what he said was right given what we knew at the time, that the iPhone wasn't going to gain much traction with that price. Apple all but admitted this with that bizarre price drop that, in fact, caught everyone totally off-guard (witness the outrage of the early adopters.)
Look, I'll reiterate: I think Ballmer is a lackluster leader for MS and probably not nearly as smart as many people assume and I think he's an embarrassing leader in a lot of ways. However, raking the guy over the coals for an accurate comment about the iPhone's price just because he didn't explicitly foresee and state an early and sudden price drop is just weird.