buying plots for cubits that was bought with real money is not pay to win, see the above quote straight from the horses mouth.
buying plots with rl money is useless on their own since you still need to gather, mine and hunt for resources as well as buying from other players, you also still need to level your characters and gain skill points none of which can be done with rl money, only through time and effort will you progress.
adding a coins for cubits feature to the exchange is pay to win and would destroy the in-game economy it could also even destroy the game itself as there would be a mass exodus of players in response to it’s addition.