By definition yes, because a B player by definition can be turned into an A player. But by that token, anyone that can be turned into a B player, is by definition a B player already. Hence, a C player, who is by definition not a B player, cannot be turned into a B player.
>I worry that C player status is something fundamentally broken about a person. Maybe it’s as simple as their intellectual capacity?
It would be motivation. Intellectual capacity can in principle be fixed, but motivation cannot because you would need to motivate them to fix it.