Why don't rudders stall at 30 deg deflection, as they don't have downwash to reduce local AOA like flaps and horizontal tails do?
At an extreme deflection (like 30º) a rudder is probably stalled. But in that condition, lift does not entirely disappear, and drag increases a lot. The resultant of the two forces (the 'aerodynamic force') pushes the tail sideways, yawing the airplane. Hence, that stall doesn't matter much...