The main appeal of tetr.io for me is it's ranked mode. You need an account, and to play a few games for your elo to stabilize, but multiplayer->tetra league is a great way to 1v1 people around your skill level
As a decently active player in the community, it's actually really hard to make a convincing bot. there's a lot of subtle tells (timing, handling, types of setups, response to pressure) that make bots really obvious to an experienced eye.
More importantly, there's just a lot of people who are really good at Tetris. Once someone practices enough, they naturally become faster because of muscle memory, up to levels that seem superhuman to beginners[1].
In short, it's far more likely that the people you played are just players who have practiced a good amount, rather than bots that have somehow remained undetected.