While you're probably right, I'm assuming that Oliver will have the middle for the Man United-Leicester game, which is essentially a Champions League play-in game. Given that the difference between the Champions League vs. the Europa League group stage is worth close to 12.5M Euros
https://www.statista.com/chart/13892/price-money-europa-league-vs-champions-league/#:~:text=A team that marches through,nearly four times that amount.), the case can be made that the ManU-Leicester game carries much more value than the FA Cup final.
I don't know how the FA, Premier League, and PGMOL all work together when assigning FA Cup vs Premier League when it comes to who has priority in assigning officials. However, I would imagine the Premier League would have a preference of who they would want to referee such a critical match (as well as the Chelsea-Wolves game now that Chelsea's safety is not assured following their loss to Liverpool), while the FA would also have a preference. But I admit I don't understand that process.