They might be saying that 1/3 of the Bible is prophetic in nature. That's not the same thing as saying it is "prophecy" in common parlance. The principle meaning behind the idea of prophecy is "forth-telling" rather than "fore-telling." Most people only think of the latter (and there is that element of it in Scripture, of course), but the prophets spent most of their time declaring the truth about God, calling the people to repentance, etc., rather than simply telling the future.