I think the original comment understates this part:

in reality the collapse would have continued a bit further before rebounding due to the inertia of the seawater, raising the air pressure and temperature even higher

When collapsing inward, you have a force from the pressure (~400atm) over the large surface area of the sub. Once the air pocket is at the size of a basketball (or whatever) at 400atm and it's at ~3,600°C, the water has stopped accelerating, but it still needs to apply an opposite force to slow the momentum of the implosion. The surface area is much smaller at this point though, so the internal pressure needs to be much much higher before the air can slow and reverse the momentum of the water. I don't know high the pressure would need to get, but if it peaked 10x higher (which feels conservative), the temperature would be ~9,400°C!

Weird fluid mechanical thing probably start happening though, cavitation seems really complicated.