Yep, bots galore. A simple Google search turned up an open source bot on github[1], meaning there are probably better bots hidden within the community.
Ugh. So many online games today are ruined by cheaters.
Being good in your household means basically nothing for an online game, especially one like Tetris. Games like this, the skill ceiling is insanely high, if you haven't seen high level play before then you're probably drastically underestimating what humans are capable of.
Other competitive game genres are like this too. Compared to 'good in my family' players for FPS games, top tier pros will basically look like they're running an aimbot. I'm big into Starcraft and the pros for that game look similarly insane when you watch from their perspective, with how fast they are at context switching.
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