As someone who plays Melee, fighting game or not (I think yes, but whatever), I'm not really convinced by the argument for Fox. It's hard to call either way for two reasons, the first beingbecause execution in Melee is so much more based on your opponents actions than it is in other fighting games, so if you assume your opponent is using inhuman TAS-like defensive options then you could easily argue any Melee character is harder to play than pretty much any other fighting game character, and the second being that this makes the combo/input system much more free form which means that at a theoretical peak Melee has incredibly demanding techniques, but the fact is that none of the stuff Fox has to do to succeed in the current meta-game is difficult enough to label him the hardest fighting game character to operate. Yes, you can do stuff like multishining shields (at which point trying to react to their buffered roll is going to be harder than the actual continuous frame perfect inputs), but how often do you see Mango (the best Fox in the world atm) even go for double shines in tournament? In almost all of the examples of Fox's most difficult theoretical tech, you're better off going for easier to execute 50/50's like shine-grab. Not to mention that this is true of almost any character in Melee, as there's just as much stuff like pivot up/downtilt which is nigh-impossible to do consistently and the options it offers have extremely high diminishing returns related to how difficult to execute they are.

The hardest thing I could easily compare to a fighting game combo I'd is absolutely "necessary" to play Fox at a top level is something like drill shine upsmash on Marth, (y/x -> c-stick down -> l r or z within 7 frames before you land[timing changes depending on how many hits and your fall speed] -> down+b -> y/x -> quarter/half circle to forward/backward -> walk/dash -> y/x -> c-stick up within 2 frames).

Even second hit-only up airs, which are pretty difficult but nowhere near the hardest thing a character has to do in a fighting game, aren't strictly necessary as you can make SDIng out of his up air much more difficult by getting deeper up-airs.

So yeah, tl;dr: Fox is hard, and because Melee is a very freeform game at theoretical levels of play he might be the hardest fighting game character, but at the level Melee is currently played at none of the stuff that is necessary to win with Fox qualifies him to be the hardest fighting game character.