i would be willing to agree on ignoring skill levels for this discussion, but your argument is like so wrong i dont even know what to say.
generally speaking, what riot means with "difficulty" in their difficulty skill stat is mechanical difficulty. sure a beginner might not be able to do rengar combos or whatnot, but honestly, everyone can easily learn to press the right buttons at the right time even in lower elos. i have some gold friends which have quite impressive mechanics on lee and similar "hard to play champs" but the hard part isnt pressing buttons. everyone who isnt physically or mentally handycapped can press buttons fast and in fast combinations. the hard thing is doing the right thing at the right time, but with this, it doesnt come down to button pressing, it comes down to how small your window of "right time" is.
there are generally speaking 3 different cathegories.
- success
- even
- failure
to have success with what your champion is supposed to do, it requires only a minimum of mechanical skill and basically everyone past bronze can acquire said skill. it also requires a certain understanding of league of legends and this is the same for all champions. you just have to know what you are doing. this cathegorie is basically the same for all champs. to win, you have to play right, simple story.
to fail is also the same thing with all champions. if the mistakes you make are too big or you simply didnt figure out the right time and place to do what you intended to do, it fails.
now the "even" part is where the difficulty of a champion is decided. basically, the larger this area is, the easier a champion. you could say, "going even" is allready a failure because you didnt succeed, but you only "failed" to a level where its acceptable, aka "forgiving mistakes".
there are certain champions which have alot of this "even" area where they can do really stupid things and get away with it while others make a slight mistake and are punished hard. this is the true difficulty of a champion - not being able/allowed to make mistakes. this depends heavily on the kit and the scaling of a champion.
auto attack based no-hard-cc champions are the ones that scale best on item advantages but they are also the ones that fall behind the hardest while at the same time having the absolute minimum space for error, since most of them can only fight all-in battles or specific trades and can only retreat victoriously if done exactly right. otherwise they will lose the trade, get pushed out of lane or even die.
also, melee auto attack champions need a certain freedom to do their work. they need to be able to stick to the target to keep auto attacking without being cced. a movementspeed or attackspeed slow alone can cripple them hard while most ability based champions have less counterplay to their damage.
surely for beginners like yourself it might seem really easy to play tryndamere and master yi because you literally just need to auto attack some other beginners who dont know what to do against it and since your auto attacks are an endless resource, you dont have to be careful with it. same as tryndamere is probably the best champion in the game against bots, because he can gapclose on them, slow them and auto attack them without cooldown or manacost until they die while sustaining up at the same time. this is a good example of how some champions may seem really overpowering at low skill levels while being incredibly hard to play at higher skill levels.
i would actually go so far as to say that the hardest to play/master champs in the game are garen, darius, master yi, tryndamere, xin zhao, miss fortune and probably some others i dont care thinking about right now.
this is not because they are weak, its because they forgive no mistakes and possess little comeback potential due to auto attacking being mostly unable to surprise people and also is pretty weak without the right items.
ofcourse you can just go ahead and keep believing champions like fizz, zed, yasuo, lee sin and rexai are super hard because you have to press button combinations but then again, most people would agree with you and you could feel good about it.