Not really, it does more than Tetris 99, and the main match making mode is 1v1 not 1v98.

It's not the only Tetris game with big lobbies. An earlier popular one is https://jstris.jezevec10.com/

It actually literally does more than Tetris 99: the record for most players playing together in one room currently sits at 670 players[1]

[1] https://twitter.com/harddrop/status/1467314109393735681

I mean, I just played a game with 100 others, and other people getting lines added lines to my board, last man standing.

Sounds pretty Tetris 99.

the focus is primarily on 1v1 (tetra league); quickplay is a way to get introduced to the game maybe but it's not the main attraction

Yup it has that mode, which to be fair existed in Tetris clones long before Tetris 99. I just meant that it had many more modes and I don't really thing that's the focus of Tetr.io? I could be wrong though I'm not a player.