I tend to believe that it is both, but luck has to do with whether or not a player clicks with the instructors at the minor league level. I truly believe that someone like Carson Fulmer could have been a good pitcher elsewhere and likewise there is a possibility that Tatis Jr never ascends if he's not traded. Similarly, I don't think that Sale becomes a great pitcher elsewhere to start his career and I don't think that Giolito turns it around if he doesn't get traded here.
Long story short, it all depends on the player and the org they go to. if the player development staff and the player themselves click, you have a good player. If it is a bad baseball philosophy fit, then it doesn't work. If you put the same group of players in a different organization, you have a completely different outcome.