Theoretically, they could guarantee it by just holding all dollars they are given originally. A price is just an abstraction for the willingness to buy and sel

Theoretically, they could guarantee it by just holding all dollars they are given originally. A price is just an abstraction for the willingness to buy and sell. If you know there are 1000 of some token in the world, and you have 1000 of some item, you can guarantee that people will always be able to buy one item for one token - by selling your items to them at that price. But of course, just holding the dollars makes it hard to profit.