I believe the bookies more than Ten Hag. There is no reason to think that United have a chance comparable to City. I think Ten Hag's words are bravado to encourage the players, and after the game he will continue his old song about the club moving in the right direction etc. By the way, I wouldn't be surprised if Ten Hag's words give more motivation to City players 
The idea of betting on City with a -1 handicap appeals to me more and more.
Bookies use data to give odds to community for bet. They are almost neutral with their data analytics in order to give odds to users. With Ten Hag, he naturally has biased thinking about issues he likes or dislikes and I agree with you completely that bookies can give us better overviews on strength comparison between two teams as well as chances to win, lose for each team. The idea of betting on City with a -1 handicap appeals to me more and more.Bookies can not be right all times but they are not gamblers and don't need to win or lose with bets in a single match. They aim at earning money from bet values both two sides and only need to balance win and loss from users against their bookies. Earning from service fee is enough for bookies in a single match and in long term.