Mo did not come out of nowhere and say "I guarantee a championship" - he responded to a question.
Stephen A. - "Now Mo, you know who you're talking to so I'm gonna give the LeBron treatment. You going to the NBA Finals?
Mo Williams - "Oh that's for sure."
Stephen A. - "Is that a guarantee? You going to the NBA Finals, gonna beat Boston and go to the Finals?"
Mo Williams - "That's, for sure. That's, that's a guarantee - we plan on winning, so to answer your question, yeah."
Closing - "You got Memphis tonight? Oh, that's a win".
Mo Williams - laughing, "haha yeah, I can guarantee that one."
There's nothing wrong with this comment. He appeased Stephen A. The guy believes in his team - good for him. What matters is what is done on the court, not what someone says on a radio show - especially to Stephen A.