Yeah… the last thing I would want a shop that had to PULL A MOTOR to find out they goofed a tensioner would be to do any more work on my truck. I would be sending them a bill from a shop that knows what they are doing.
In the TDI world timing belt mishaps were the main cause of engine failure due to it being an interference engine.
Now, this valve to piston contact could happen a lot of ways, but here are a few:
Mistimed then cranked over - immediate bent valves
Camshaft bolt not torqued properly - bent...