Perhaps an explanatory note:-
True air speed (TAS)= Indicated air speed (IAS) only at sea level in a standard atmosphere. As density altitude increases, a given IAS is equal to a higher TAS.
Thus, whilst rudder effectiveness decreases in TAS terms as altitude increases, it remains constant in IAS terms.
Thus, whilst an increase in altitude may have an equal effect on both power and rudder effectiveness (in TAS terms), the IAS at which sufficient rudder authority is available will decrease.