It would be difficult to over tension the belt because the belt tensioner on most late model Mercedes engines are spring loaded. The tension, or preload, is engineered into the device. I would be more inclined to think that it was dumb luck mixed with a technician who was interested in his flat rate time! It is possible that the technician allowed the hub holding tool to rest against the water pump housing while removing/reinstalling the crank bolt. Bad practice, seen it a few times. A well seasoned, tuned into his work technician is a rare commodity in most if not all dealerships nationwide.

I am sorry to hear about an experience that appears to have been avoidable.