2001 Skoda Octavia Damage caused by badly fitted timing bel

I had the timing belt in my diesel Skoda Octavia changed last year by a local garage. Subsequently it started to lose power and accelerate slowly. The engine then failed completely and I had to get a new engine fitted.

The garage claims it cannot have been the new timing belt because only one valve in the cylinder failed and dropped through into the cylinder. They claim because none of the internals of the engine were disturbed it could not possibly have been the timing belt.

Is this true or can an incorrectly fitted timing belt cause just one valve to fail?