Their financing makes money on the differential between the rate they pay on demand notes and the rate they lend to car buyers. Anyone (well, US Persons at least) can buy the demand notes [1], which are currently paying 4.75%. So as long as they are lending at an average rate above 4.75% after defaults and other problems I guess they are in a good position. But they can also move money around internally, I suppose, to make lower rates possible.
[1] https://www.ford.com/finance/investor-center/ford-interest-a...