You also have to consider a few other possibilities, such as neither side leaked it, the source was pretty unreliable. or the (IMO more likely) possibility that she doesn't have a source and was merely expressing her opinion to some colleagues.
Wonder how your opinion on her would change had she said something along the lines of "Rondo loves Boston and plans to retire there. Him re-signing is a formality. His teammates and coaches, both former and current, love him and admire his leadership and maturity".
I'd probably say that was fairly over the top, and that she must not have talked to Ray Allen. I'm probably a lot less likely to blindly accept anything I hear that supports my position than you are.
Beyond that, though, it's worth pointing out that we've seen plenty of quotes from Rondo about how he's open to staying here and plenty of quotes from his teammates, both former and current, about how they like Rondo and think he's a good leader, while we've seen none where he says he's not considering coming back to the Celts. That's something I'd consider when I was deciding how accurate Jackie's claim was, as opposed to your near fool-proof method of believing whatever rumors you agree with.