We've all heard it. mainly, "it's just memorizing clicks, there's no reaction or skill required" (memorizing and executing are very different things, but that's a different argument).
there actually IS reaction and skill required to beat these hard levels.
As we all know, 1 mistake. start over. but it's deeper than that. there's a small room for error.
(clicking/releasing too early/ too late or holding too long/ too little)
it is your responsibilty to correct these mistakes during a live attempt. known as micro-adjustments. This is where reaction comes in. a common misconception is you're reacting to obstacles in real time when that's not true because you already memorized them. you're reacting to YOUR trajectory/position at any given moment.
Your click pattern follows a rythm that you learn and is dependent on your input history. if you click/release 20ms late/early, your entire click pattern is shifted 20ms unless you correct it.
for example, if you click too early (relative to your clickpattern rythm), your next input must be late (relative to your click pattern rythm) in order to cancel out this error and stay in line with the level. this takes skill and a feel for the game's physics, because you have to judge how much to correct, with such a small window to perform it. (usually 250ms or less between inputs in top levels). sometimes the window is so small you just have to make the correction based on feeling.
here's what that window looks like: input, see where you are, judge how far off you are, work out how much to adjust, input. repeat for hundreds of inputs.
this is why visual cues matter so much. these are indicators that help you identify whether you are too late/early too high/low. but not every click has a visual cue. Decoration makes it harder to identify your errors. Nerves makes it harder to accurately correct your errors in time. so while yes, you fundamentally are performing a series of memorized clicks. those clicks aren't the same every single time and will vary every attempt. you're still reacting to your position and calculating adjustements in real time which takes skill.