I think the idea is that RP is a sort of colour commentary that runs in the background and flows from the "hard facts" of the game. It can't be used to create hard facts.
In the sense that you can't use RP to change game mechanics, that's correct. You can't, for instance, RP that you cut off a man's sword arm, and then expect his skill at swordfighting to be adjusted accordingly.
However, the main point here is not that you can't cut off another character's finger. It's that you can't do it
without the player's permission. Nobles can certainly have disabilities; the previous Queen of Luria Nova was blind, for instance.