On one hand, he named two people within F1 who he'd told, so it's going to be true, but on the other hand, he's trying to sell books, so how much impact did it actually have?
Everyone expected Vettel to move over had he been ahead of Webber and Vettel himself was unable to win, so while the two sides of the garage weren't exactly friendly, they were going to play the team game if it was needed. What's happening in Webber's mind is interesting though - he must have assumed that the team would have instantly put all their effort into Vettel's car, and he didn't need another reason for them to do so.
It may have coincided with Vettel getting the upper hand again, but nothing changes - Webber lost the title when he sideswiped the wall in Korea.
I guess he's not offering it as an excuse so much as publicising his book. We all know he's got a book coming out now - before I had no idea.