fox is pretty tech heavy, but more than that, he requires good tech to play.
Did that sentence confuse you? There's tech like techniques and then there's just general tech skill and input performance.
Fox is hard to play properly at a high level because he requires an insane amount of input performance, even the simplest inputs have to be done just right. Timings are SUPER important, you mistime something by even the smallest amount and it can be the difference between you hitting them and them hitting you, which can be the difference between you following up and getting a kill and winning, or them following you up, killing you and winning against you.
Precision is the name of the game, even the simplest things have to be done with absolute precision and timing.
Foxes character is build around this, and its even more important because fox is so light and actually kinda gimpable in certain situations, so every mistake you make costs you so much, much more than it does with pretty much every other character.
In a high level match, every tiny little mistake you make with fox is a potential kill for your opponent.
"Whoops I was panicking and in the heat of battle I accidentally usmashed instead of utilted, instead of my utilt catching them and giving me an uair follow up to kill them, they were able to punish my usmash end lag with a kill move, instead of winning I just lost, because of one tiny little error I made under pressure, ****" - actually happened to me :<
practicing the simplest things can really go a long way to helping you tighten up your play, especially in high pressure matches where you will make way more mistakes than usual.
And by simple things I mean things like, hold shield, then drop shield and quickly turn around and jab/grab, do it as fast as you can without accidentally rolling, or jabbing in the wrong direction, or inputting a dash instead of just turning around, all simple mistakes that make a huge difference in what you want to happen in a match. Precision is what its all about.