well, no it doesn't if he goes to play under Pep or Mourinho - or even at Barca...
Of course you are right but I am not so sure that either of those 2 great coaches have improved individual players as defenders.
What Mourinho would do is coach Stones to play as part of a collective - which is much needed.
Pep's teams play a high pressing game and he encourages His defenders to play out from the back - which Stones loves ( but Mouriho disdains ).
So yes of course his game would / should improve under either of them but I suspect that Koeman having played the position would show greater interest / empathy in developing Stones individually.