The car will be guaranteed, except to the extent that nothing in life is guaranteed. If the prior renter of the Prius doesn't return the car on time, and there are no others, well, they can't make one appear magically.

And FWIW, a few years ago Hertz had a program where you could reserve specific models at most major airports within the base categories (e.g., mid-sized, full-sized). It quietly disappeared...