I’m guessing dummy.bin is a zerofile?

Ugh. MS-DOS 2.0-5.0 inclusive didn’t have a defrag tool. It was introduced with dos 6. While it could be helpful, the fact that we went more than a decade without a defrag tool as part of DOS reinforces just how optional it was/is. The benefit of defragging was that it would be marginally faster to read a file that was stored contiguously instead of in pieces. There was the side benefit as well that it was easier to recover data that wasn’t fragmented.
I’m not aware of any legitimate ‘Problems’ caused by simple fragmentation itself. That tech guy was not just wrong in his behaviour, but also in his technical knowledge. What an ass.