Generally that's true, but Hertz specifically does have a few classes of vehicle that only contain one model, and they advertise those as something like "guaranteed make/model" / "reserve this exact car".
And while they do run out, most of the time you will receive what you booked, and at larger airport locations they'll typically substitute something close in size and nature to what you booked if they do run out of your specific class.
And while they do run out, most of the time you will receive what you booked, and at larger airport locations they'll typically substitute something close in size and nature to what you booked if they do run out of your specific class.