Sandy Farquharson says playing stronger opponents sharpens your tactics and shot selection because they punish mistakes weaker players ignore. But technique won't improve on its own. You need to diagnose and fix technical flaws alongside the tougher competition, or you'll plateau.
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Playing better players is great for the tactical side of the game because you would play and not get away with stuff that you would otherwise get away with, with lower level players. And so what that means is when you might get an easy smash, for example, you smash it and a low-level player, they don't run forward, but a higher level player, they do run forward and they put you in trouble. And that is a good reminder of the shot selection and the tactical side of the game. So I'm not saying playing better players doesn't help at all. It does. You can improve your tactics and understanding of the game, but what you find is that you don't actually work on the technique. And so it's quite difficult to dig yourself out of a plateau by only playing better players. You really need to get to the root of it and understand what is going on with your technique and tactics if you actually want to improve.