Tinstafl,
You recall correctly.
Because the turn coordinator gyro is oriented off axis it can pick up roll as well as yaw. If you are rolling in the same direction as yawing (which is the case in an erect spin) then the indication is unequivocal.
However, when you are rolling and yawing in opposite directions then the results are unreliable because it would depend on the predminant effect. Thus, in an inverted spin, the indicated direction can not be relied upon.
CPB