I can barely even register the next piece at that speed, I wonder if these players are mostly just visualizing the board in their mind. Doesn't seem possible to be looking at both the upcoming piece and the playfield at this kind of speed.
The bottleneck isn't the keyboard input. Pretty much all top players use ARR 0 and 2-step finesse[2] (or something close to it), which means you can position any piece where you want in 2 keystrokes at most, plus 1 more to hard drop it. And certain stacking styles (e.g. 6-3 stacking[3]) lend themselves well to not requiring the full 2 keystrokes pretty often, so in practice you end up with ~2.6 keys per piece instead of 3. A typical DAS for a pro player (e.g. someone like Firestorm) might be somewhere around 70ms. So if we roughly estimate 100ms average per piece (70ms to DAS, with some extra for the sometimes-needed rotation and hard drop), that would be ~10 pieces per second, which is well over the ~6.5 PPS in the record sprints.
The physical pressing of the keys also isn't really a bottleneck. Keys per second in the top sprints is ~16. I just mashed my movement/rotation keys into `time | wc -c` and got ~35 KPS after 2 beers, and I don't even have a particularly gaming-focused keyboard.
The bottleneck also isn't reaction times. Someone might trot out a hastily-googled 250ms as a typical human reaction time and claim 100ms per piece isn't possible, but good players aren't reacting to each piece as it comes -- they're using the previews (and hold) to react to the piece that's coming 4-5 steps in the future, which is closer to a second away.
It's really just about how fast you can process the upcoming queue and still stack cleanly.
[1]: https://harddrop.com/wiki/DAS [2]: https://four.lol/mid-game/finesse [3]: https://harddrop.com/forums/index.php?showtopic=2985
I can see now how this becomes a much more organized and deterministic game with these changes, allowing players to plan their moves and inputs precisely so that they are barely need any visual feedback to confirm the state of the board, allowing them to queue up multiple moves in rapid succession.
Of course I’m sure it takes a lot of training and practice as well!
That's because the NES Tetris has considerably different mechanics. NES Tetris simply wasn't built for fast play. The DAS, piece locking, RNG rules etc. are very unforgiving, so the players are doing most of the work fighting against them instead of finding piece placements as fast as possible.