Viking50 hasn't been back since the 8th.
Presumably the hole was in a location where holes are typically reparable. The fact that Viking 50 is so confident that it was reparable suggests that it was in a location that is known to be an easy fix. Probably on the tread, and not on the side-wall.
Some reasoning may have been provided by the store, but this was not given in the original post. There might not have even been any reasoning provided by the "expert" at Crappy Tire - just a simple statement: 'not repairable'.
The real reason, most likely, is that the store would make more money selling two new tires, then they would patching one hole. Maybe even sell 4 tires, if they were persuasive enough.