Why should the caller have known better? He knew better after Rav Moshe told him.
The caller, as I heard the story, was Rav Shmuel Berenbaum, who went through a sugya for many days, and asked Rav Moshe a big kashya, which Rav Moshe immediately answered to Rav Shmuel’s satisfaction. He told Rav Moshe it was good he had just learned the sugya, to which Rav Moshe responded that he hadn’t learned that sugya in years.
He was so impressed by Rav Moshe, that he called him again, and then again, at which time Rav Moshe told him that t he reason he gets up so early to learn is that this was the only time he gets to learn uninterrupted.