Bishop and rook pawn on the seventh hold against a rook when the bishop sits in the corner behind the pawn and the king protects both. The attacking king is tied to the promotion square and the rook has no target.
A rook normally beats a minor piece and a pawn, but rook pawns on the seventh create fortresses. Black's pawn on a2 is protected by the king on b2; the bishop on a1 is protected by the king and blocks nothing important, since the pawn's promotion square is... occupied by the bishop itself. That is the point: Black does not want to promote, Black wants to stand still.