Joining the Tour with worse form than last year and believing you can deliver is overconfidence.
Thats just how you interpret it.
I dont think he had much of a choice. The plan was always going to the Tour.
His crash and the injury he suffered was a major set-back, but not enough to rule him out. They stuck with the plan and was gonna hope for the best.
Everyone wanted another duel between Vinge and Pog. In some ways his participation is way bigger than even himself. It is key for the sport, the spectacle and the race. Even if he is only at 95% or if he would have been at 99-100%, which still might not have been enough given Vingegaards peak form that that he is in atm.
Saying in the media or interviews beforehand that he dont know where he was at, or he that he would have had a small chance of winning would have been to concede before it even starts. That would have been a terrible build-up towards the race, if his crash and injury already wasnt. He probably shouldnt have started at all, if he would have said those things in public, but that is not what seems to be the character of Tadej. Hence why he probably tried to be somewhat up-beat and positive regarding his chances to not kill the race before it even starts, for many different reasons. It is not over-confidence. How you would feel that way or want to say that is what it is... is probably your own bias coming into play more than anything.
Vingegaard was monstrous today and is likely to win it, but anything could still happen. Ask Simon Yates. Ask Roglic. Things can happen in a race, where someone is looking unbeatable.