I understand what OP means, but it isn't pay to win. Pay2Win is when you can buy it DIRECTLY from Cash Shop.
Selling NCoin isn't Pay2Win. It's....well. Credit Card hero. I know P2W people use their credit cards too but, idk, I like to think it's slightly different. Selling NCoin is more like Pay4Convenience? Pay2GitGud? Because the Skillbook is available in game, not cashshop. Therefore, you're not PAYING NCsoft, directly. The Gold you buy with NCoin has to come from the game ONLY (from other people's farming).
TL;DR: NCSoft isn't selling Skillbook, not pay2win, it's p