Obviously if a team like City come in for Ten Hag at the same time as United then he may well opt for City, for reasons that are all too obvious.
However if it's a choice between joining United immediately or the promise of joining City a season later, that obviously skews the odds more in our favour. Because a promise without a contract could be reneged on (if Pep changed his mind, for example) and all else being equal a team that will hire you now is better than a team will will hire you in a year.
Obviously that doesn't mean he wouldn't hold out for the City job, it depends on his priorities and his read of the situation. But saying he definitely would/wouldn't either way is just baseless speculation, it depends on him and his own personality. Some people would opt for one, some would opt for the other. Klopp turning us down but accepting Liverpool wouldn't have been what the forum thought was the on-paper correct decision then either, but he had his own opinions.
There's also the fact that he may not even be City's first choice, as they may opt to go for a post World Cup Enrique. Hell if Enrique was currently available most of this forum would probably want him over Ten Hag too, we've just taken it as read that he's staying where he is until after the WC.