If TUI are getting extra aircraft at short notice then yes it makes sense i'm afraid - if you promote 50 TUI FO you also need to recruit 50 FO to replace them and 50 for the extra aircraft.
TUI are taking advantage of TC demise and why shouldn't they. The thing TUI must do however is keep promoting FO's for the current establishment such that the current (loyal) FO's are not discriminated against. I'm sure BALPA have put this message over strongly. They should also say that should any of the 50 DEC leave then they are replaced by TUI FO rather than more DEC such that the (loyal) FO's benefit again.
This is NNUTS - Nothing New Under The Sun - many other airlines have done this before