I dont agree with that, if theres one thing that's impressed me about De Gea this season it's been his attitude and mentality, which has been excellent for such a young player. Given the treatment by the media at the start of the season who seemed determined to make him out as shit and pick up on any mistakes he made, his confidence and self belief has been totally commendable. Theres many players who would have had there confidence shattered after the games against City in the charity shield and West Brom, as he was being painted as a bit of a flop already. Even amongst United fans some were questioning his ability and Fergies decision to buy him.
If he was in any way a confidence player then it would have shown in the following weeks, but it didn't. He came back and put in some good performances, the made a few more mistakes, but carried on regardless and generally looked impressive. He's nowhere near to being a confidence player in my opinion, in contrast he seems pretty oblivious so criticism and gets on with playing football.
The mistakes he's made aren't the result of a lack of confidence, they just seem to be happening due to him being a young and currently inconsistent player. In the same way Jones has made a few mistakes as a young player, they've both made themselves available for the next game and tried to do better, which shows self belief.
I also think it's telling that Fergie didn't drop De Gea when he was making mistakes, especially as there was a ready made substitute in Lindegaard. If he was any sort of confidence player and Fergie thought it could impact his performances, then he would have sat him out for a few games.