I agree.
Until we've determined whether the odd ball is heavy or light, the number of possible outcomes must be even: that is, 2 times the number of suspect balls.
To render 26 outcomes to a manageable level with two weighings to go, the first weighing would have to present three cases of 9, 9, and 8 possible outcomes.
One weighing cannot determine the relative weight of the odd ball. So it can't yield a division of cases containing an odd number.