Exactly brother Ayman. If he was an imposter or faking the whole thing, he would have been exposed by his own people and supporters. The Sahaba would not have risked their lives, wealth, familes and property to support a fake. Furthermore, this secret teacher would have been exposed by the enemies or even other Muslims. There are al kinds of myths creaated by non-Muslims. For example in my parennt's culture, they have a rumor that Bakhira the Nestorian Monk was the author of the Quran, and they accuse Muhammad(saaws) of killing him later and throwing his body down the place where Muslims throw the pebbles during hajj, lol. Subhan Allah I was told this and believed it during my entire childhood. This is one of the reasons I have not been able to give any dawah to any people of my ethnic group because this myth and rumor is established among them. They won't even listen and the moment I try to refute them or give them dawah they immediately stop me and say "we already know the truth about your prophet".
They say nasty things and I do not want to lose my temper or do anything that i will regret or make me look bad as a Muslim.