I'd respectfully disagree with that. The analogy I'd draw would be to be Christian and unaware of the doctrine of the Rapture - there are numerous Christian denominations who either don't use that term, or who do not believe in a literal Rapture.
In this case, the Mahdi prophecies are taken the most seriously by the Wahabis and the Shi'ites. The "orthodox" Sunnis don't take them seriously at all, though many of them might agree with the stated goal of establishing a worldwide Caliphate.