I have a book that was put on a 48-hour wishlist hold on the 18th, but now it says it was reserved today.  If the person doesn't confirm that they have the credit for the book, does the site keep on extending the hold, or am I misunderstanding something here?

It sounds like it flipped over the next person. When someone declines a WL offer or lets it time out it goes to the next person in line or on your shelf if there are no other wishers. So it's been offered to the person who was 2nd in line behind the first person.

After 48 hours, the book rolls over to the next person on the WL.  So, either the first person finally confirmed they want it or it went on to the second person who either confirmed they wanted it or had it on auto-request.

What they said.