Actually, I find that one completely believable. Never forget, for all his faults as a manager, Michael is an absolute god when it comes to sales. He got sent to Winnipeg to try and woo back a customer, and closed a two year deal. Bosses will put up with a lot from a top performer like that. And just as importantly, Michael chewed out David in private, over the phone. If he had done it in front of the entire Scranton staff, then yes, David would have had to do something. But by doing it privately, David can let it go as Michael venting.