No, he hasn't.
And I think he wants a new contract at United now. Which he should get.
That said, the original "new challenge" comment didn't make much sense. He hasn't achieved anything of note since he returned to United. Which certainly ain't his fault exclusively - not by a long shot - but still: you normally move on to a new "challenge" after you have completed one. So, unless he thought being a bit underwhelming in a very underwhelming team amounted to completing a "challenge", again - the comment made little sense. He should've said (if he wanted to be perfectly honest): "Perhaps it's time for me to move on from this shit show".
Perhaps he was just being diplomatic.
Anyway, water under the bridge, etc.
You are over analysing the semantics. If a player in the last 5 years joined Arsenal to ‘win titles’, should they just stay there until they retire and keep trying? The overall point is that the project and challenge he came for hadn’t come to fruition, and did not look like it would, not in his time.
As for those who keep saying he has a part to play in it not working, that doesn’t matter at all. All that matters are his options. Whether you think he deserves it or not, the facts were that if he left United last summer, he would leave to a team that WILL challenge for trophies. Which means that is the choice he has. Stay at United and not challenge, or go to Italy, Spain or France and compete. I don’t know in what world players are supposed to think ‘well, I have played a part in this so therefore....’. Therefore what exactly? Should a relegated player, like say Jack Grealish if Villa went down, not say he wants to love to a PL team because he has to shoulder some blame for Villa not being good enough to achieve their goals? Or Nathan Ake and a host of others who will leave their relegated clubs this summer to stay in the PL, despite contributing to relegation.
All that matters is that they have suitors who think they are better than that. And Pogba did/does. If he were so bad, he simply wouldn’t have the option to say he wants to leave United and go and play for Juve, Real or PSG. But if you can do better, you would do. If Ake couldn’t, he’d still be at Bournemouth.
The ‘loyalty’ from a lot of the players in our squad is borne out of the fact that they are being made to choose between playing for Manchester United and playing for Newcastle United (at best, in many cases). Give a player better options, and we will see how many years they would be happy to remain 6th in the league. I said yesterday, but personally, I want the very best players to get itchy feet if we are underachieving for long periods. If they don’t, then that indicates to me that they aren’t really fussed about winning, and will gladly spend their prime years outside the CL so long as they are being paid handsomely. Ozil, for example, showed where his motivations (and options tbh) lay when he signed a mega deal at an Arsenal that appeared to be going nowhere. De Gea too. And both are showing that they have lost a step. Them looking to go and win trophies may have at least suggested that they had genuine hunger left, but both were bribed to stay at poor teams for record wages. That’s not what you want as a fan, I don’t think.