ROR went from a guy who could barely shoot or tip a puck... into one of the best goal scorers in the league from in tight. Tips, bar down, quick moves. He never scored 20 goals in the OHL, but has 8 20+ goal seasons in the league. Marchand was a pretty mediocre scorer in the Q. He was a first line guy there, but you never really saw him as a top line NHL guy. 3rd liner probably, maybe 2nd liner if things clicked. Even his first few years in pro, that is all he looked like. But he worked on his game, became a better passer... and more importantly to his point, he became a threat shooting the puck. This transformed him from a 2nd liner to a high end 1st liner for a few seasons. He's certainly on his deline now... but 28-34 (which is an odd age to peak), he was fantastic.
There are things that can't be taught or worked on, but most of hockey is actually things you can learn and technique. Shooting and hands are technique and repetition. There are some muscles that you need to be elite, but they can be worked on even if you don't have genetic gifts to get them to more acceptable levels. Guys may not be Laine in shooting or Zegras... but passable in the NHL is there for almost anybody with enough reps.
What I don't think can be taught is just the freak athletic ability... so things like skating, explosion, bulking up without slowing down, etc. Yes, you can refine areas and work on things to get the most out of your body... but there is a limit. ROR is one of the hardest working players in the league, and he'll never get beyond his limitations athletically. He's just not blessed with MacK's athletic gifts. That's why I don't think skating is all that easy to fix... you can get all the technique and prettiness in the world, but if you aren't athletic... you're never going to be elite. See Jost. If somebody is sloppy technique wise, you can that... and if they are athletic, you can elevate them more.
