No, it isn't. There might be funding limitations, but if the demand to become a doctor is too high for the number of positions offered, then they can stretch the funding over more slots by passing more of the burden onto the resident in the form of lower pay.
To the extent that they can't legally lower pay, that is the factor, not the funding.
The argument that medicare funding slots is the limiting factor is like saying that Hamilton can't come to LA because the NEA won't cover airfare.
An interesting post on this is here, although it somewhat contradicts the popular notion that graduate medical education is limited by funding: https://theincidentaleconomist.com/wordpress/gme-federal-fun...