Things I hear are a bit different. Of course his engineers can't go to RB. They don't know anything about that car.
Problem seems to be completely inside the team. There seemed to be a Sainz side and a Verstappen side there. It is being said that Sainz' engineers have asked things the Verstappen side didn't like and the same stuff has been done to the other side. Last thing was the Verstappen side with Pujolar not adhering to the TR tactics in qualifying in Russia. Pujolar didn't follow up what Tost wanted or something like that.
In other words it seems Verstappen and Sainz were having a conflict which was completely internal in the teams as well, and they can't drive together in the team as that brings too many problems. Verstappens race engineer is also a Catalan, while Sainz is a Castilian. Also something that's not working fine. So there's a reason why Verstappen had to go to RB and why his race engineer has been fired.