You are right, they typically need at least two sources to confirm a story. In this instance, I think they probably did but Brady, et al did not want to do so until after the SB. It may have even been by request of the NFL that he wait so putting the kibosh on the story right now is the only way to do so.
I'm willing to bet that he is retiring but wants the spotlight instead of taking away from or sharing it with the playoffs and the SB. I can see no good reason for him to play a losing season next year in Tampa.
