I really don't understand this sort of attitude. He was a boyhood Everton fan. Why do you think that should have changed? Surely everyone knew he went to Utd to win trophies, which he did. Pretty much every footballer ends up not playing for their boyhood club. Not sure why you think his remark is trashing his Utd career.
Of course Everton is his team as a supporter as well as a forthcoming second stint.
Professional footballers are employed by many teams over their careers and" kiss the shirt" when they start at a new club. They will still have loyalty to the team they support (as do all supporters) but his "honesty" is a little naive in my opinion. He's basically saying that over the past thirteen years at Man Utd where to say the least he's had a good career and achieved many things and won many trophies but he's trashed that in my opinion by saying his heart was in Everton all the while he was turning out for Utd.
I think it's disrespectful to Man Utd who provided him with his career and enabled him to win trophies. (Though he's saying what a lot of players privately think).