That means that pre-authorization does not guarantee you will receive an iPhone on the 25th. It is on a first come, first serve basis.
Is pre-authorization the same thing as making a reservation?
It still says that my reservation will be held until the close of business on June 24.
Yes.
But, if they are out of stock when you arrive you will not be able to receive an iPhone on that day.
So why did Apple accept reservations at all then. I've never heard of a business holding a reservation for a day that the product or service may not be available. Could you imagine reserving a table at a restaurant just to arrive and have them tell you that they won't have a table available the entire day?
The reservations are held for people so you get your iPhone before the general public without a reservations, but it is still on a first come first serve basis.