That is true but rudder has a much greater effect.
But what exactly do you mean by that? That if you allow the aircraft to slip out of the turn by failing to keep the ball in the middle, the aircraft also tends to roll out of the turn? Yes, but that's the effect of the lateral stability (provided by dihedral or other stabilising effects) -- the aircraft wants to roll out of a slip. What the rudder is doing is eliminating the sideslip. That sideslip can indeed have a substantial impact about the roll axis.