It's wholly up to the carrier to have ALL the resources in place to honour their commitments - something the CAA used to patrol rigorously (I can remember someone having to end up subchartering in a 737 to cover a half-full- F50).
It has been obvious for a while (since at least January) that travel restrictions would be lifted this summer. It should not have been a surprise to a tour operator that people want to go on holiday for the first time in two years.
At what point can a case be made that TUI (and many other travel operators) were selling tickets that they had no realistic hope of being able to deliver?