A man who genuinely knows he is nothing can boast without danger, because there is nobody home to be inflated by it. The boasting is theater, done because the crowd demanded credentials, and he says so while doing it. That transparency is the difference between ego and the absence of it.
Being nothing is not humility, which is only pride standing on its head, arranging to be admired for refusing admiration. It is the plain fact discovered when the false center dissolves and comparison loses its ground. From there, claiming equality with the greatest and claiming nothing at all are the same statement, and neither one costs him anything.