This is only true when playing with others attempting to play optimally. If you only play optimal poker it may be difficult to take advantage of sub optimal players without modifying play.
A simple example is a rock paper scissors tournament. The nash equilibrium says you must always simply pick each with 1/3 chance. But if you play this you are almost gauranteed to finish in the middle of the pack. The winner is going to be someone who can take advantage of other players. There used to be a rps AI tournament years ago (UofAlberta I think), and this is why they wouldn’t take submissions that only played nash equilibrium play.
But you are right, in that usually the first step in getting good at poker is to simply play the strength of your cards based on your position. And this becomes more amd more important the higher the stakes since players tend to be better there.
