Take a game where you get to pick a single number between one and a billion. If you pick 10 you win. This has a billion states, but it's trivial. I can increase the bound above a billion, it doesn't matter.

State count gives an upper bound, though, to how complex a game can be, for sure.

Those position counts don't account for blunders or bad tactics which the players will take advantage of in order to simplify and win. In go, any move that lets your opponent cut you off is highly likely a blunder, so counting every other move other than the obvious one you have to play in order to not lose is pretty useless. You can also call it a bad move if you are not under an immediate threat and you are not invading. (this is a guess, I am not a go player) So counting the rest of the moves developing in your territory is also useless.

Yes, for example no limit Rock Paper Scissors where you bet an arbitrary amount and your opponent might call or fold is a game that:

1)has infinitely many legal states/positions

2)imperfect information (another argument often used to argue that game is more complex)

And is:

3)dead simple to play optimally

It's also easy to design a board game with arbitrary number of legal positions that is dead simple to play optimally.