Most likely scenario and simplest explanation, especially given Hertz known IT issues and many other reports of incorrect mileage along with consumer hostile treatment - Hertz had the vehicle mileage recorded wrong at the time of rental.
Threatening the renter with police involvement is way over the top and the Hertz rep is entirely out of line and wrong... as is usual with Hertz.

How 25k miles supposedly got on the vehicle is another story and seems extremely unlikely, which reinforces the theory of the mileage being recorded incorrectly at time of rental. Even as a ride share or commercial use it is highly unlikely to hit that kind of miles in a month from actual driving due to the combination of continual use and speed required to hit 25k.