The administration is essentially arguing that IPAB will cut costs only by reducing provider incomes, not by curtailing in any way the consumption of Medicare beneficiaries. This is possible, I suppose, but it is not supported by either economic theory, or historical evidence. (And indeed, the early discussons of health care reform, as well as my interactions with the administration's very smart economists, make me suspect that they, too, believe that IPAB will curtail service provision . . . but also believe, correctly, that saying so would be political suicide.)
The "fact checkers" have thus somehow annointed the least likely outcome as a "fact" about the future.
This is, as others pointed out during the welfare kerfuffle, the great problem with fact checkers. They have no particular policy domain knowledge, so when the administration tells them that well, the law explicitly forbids IPAB from rationing treatments, they are in no position to understand that this doesn't really make any sense.
There's nothing wrong with opining based on the information you have; the problem is with calling the results a "fact". The even bigger problem is that other journalists then treat it as such, transforming a shallow understanding without roots in history or theory, into a known thing, no different from stating the color of the sky or the height of Mount Rushmore. Then, of course, they're free to declare that anyone who disagrees is lying.