Then there's the fact that it just takes one skilled bender to knock the mask off in front of people and reveal the lie, if he is lying. What if one of the benders he had let fight him got lucky? Poof, there goes his credibility. It would then make people wonder if he was lying about everything he said and just like that he's lost a whole crapload of his support.
Of course he could be lying, but for now I'm going to believe him about his story until we're given any reason to think otherwise.