It's not clear that he was buried in a mass grave, although that was implied in the movie Amadeus. The article says that he was buried in a pauper's cemetery. I saw a picture of the skull on the cover of an archaeology magazine about 15 years ago. Apparently it was dug up by the person in charge of the cemetery, who would have been is a position to remember its location.
Yes... however paupers were usually buried in unmarked graves which were often in the same hole, side by side, or even on top of each other. Essentially, you have a mass of dirt and bones. If there was no match, the man dug up the wrong skull... as there were probably many others in nearly the same spot.
If it was dug up 15 years ago and the person remembered the location in which Moz was buried, he must have been very old.
I used to work with a guy who was a gardener and grave digger at the local cemetary. The grave sites had multiple graves piled one on top of each other.
They used a backhoe to get down to the casket and then he would finish digging by hand.
He occasionally found a bone and took it home to feed it to his dog.
ISYN the world is a strange place and you can't trust a grave digger. The job makes you a little bent.