Racing Kings

Racing Kings players: why can't black move the rook on b2?

lichess.org/variant/racingKings

"In this game, checks are entirely forbidden: not only is it forbidden to move one's king into check, but it is also forbidden to check the opponent's king."

I know, but the rook should be able to move to b3 or something right?

Discovered check from the queen on a1

oh thanks @asdf5656