Are you saying that even an amateurish pianist who has absolutely no experience in building up endurance in muscles can easily pull off a fast Chopin Etude so long as he/she has the right technique for it? cuz That sounds kinda unrealistic for me.
For what it's worth, I can play a considerable part of the Revolutionary etude without feeling tired, and haven't been playing for that long. I also shifted to playing on grand pianos this year, and I was able to play the first half of the etude within a month after the switch. I had been playing on keyboards and light digital pianos for about five years. I don't really get tired at the piano usually, even feeling lactic acid burn is quite rare for me.
That said, I have asked a pianist about it, and he said he could see that I had finger strength. So maybe I had acquired it over the years. However, I don't think I ever truly felt tired at the piano. And this is coming from someone who used to do 4-6 hour marathon sessions, even near the start of my piano playing journey.
However, when asked, the pianist answered that people who lack the finger strength are unable to play fast without clean articulation. That is, they can play with clean articulation, they can play fast, but not both at the same time, because clean articulation at that speed depends on how nimbly and precisely you can control the tips of your fingers. Which depends on finger strength.
What exactly does it mean? Firstly, I've heard that it prevents your first and second knuckles from collapsing while playing, especially in your weak fingers such as the pinky. I'm slightly double jointed, which is especially apparent when it comes to the pinky and ring finger, which sometimes leads the middle knuckle to collapse backwards while playing something loud with it. Apparently, that gets better as your finger strength improves. Also, I've heard that your flexors become stronger and get better at fast twitch movements.
It's kind of scary for me to think about finger strength, because it seems to imply that there are certain anatomical changes which happen while playing the piano when you're taught at a young age, which can't be learned later (and several people have told me so, in as many words). Thinking about it purely as a coordination issue is more comforting, because that means that it is "only" a matter of acquiring the right movements and learning to not get tense.
It has been my experience as well that the internet seems to reinforce the point that finger strength does not exist. However, most professional pianists I know and have talked to have said that it is very important. And I have changed my opinion somewhat. I've observed that most of the people who talk about only using body weight don't play challenging virtuoso repertoire. That makes me skeptical. It's possible that lack of finger strength only makes itself apparent when you're playing difficult material, and I am inclined to believe that. As I said, one theory I've heard which makes sense to me is that clean articulation at high speeds involves finger strength. I can play fast without issue, but I've generally received the comment that my playing isn't that "clean", the reason being that I don't have, or haven't learned to use finger strength.
Apparently, European schools put more emphasis on finger strength, which checks out in my experience so far. Again, it's not quite clear, but I vaguely hear that it is related either to the strength, or the flexibility of the flexors related to the fingers, and even the interossei. Often, these would be considered to be weak muscles which are irrelevant and approaches which emphasize body weight and ergonomics often completely ignore three small muscles in the fingers. However, I have heard time and again from proponents of finger strength, some of whom are concert pianists, that is simply not possible to use body weight to play very fast passages cleanly.
Check this out at around 1:05. He's provided English subtitles. Basically, he talks about being able to grab the keys as hooks, and this is similar to what I've heard elsewhere. Initially I was very skeptical, but I'm gradually coming around to believing it.