Not universally true. I know that once it's put out on the buffet it can't be served again. I imagine there's a limit on how long, once cooked, food can be kept at temperature until it has to be thrown out. But it's not unusual to see food on the buffet that's obviously leftovers or somehow re-imagined from the last night's MDR menu.
For example, there is often beef stir fry on the buffet. I've noticed that the kind of beef used varies, possibly depending on what MDR beef dish was on the menu last night.