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.

[1] happy to 1v1