Engine anti-ice is done till -40 deg SAT, because you will not find any undercooled water anymore below that temperature. Below this -40 deg SAT iceforming is highly improbable.
This procedure is only valid for high-powersettings (climb/cruise), so in icing conditions (below 10 deg TAT and visible moisture) in descent you will switch on engine-anti-ice, even when colder than -40 deg.
To my experience, most wing-ice builds up in the region between +5 and -5 degrees TAT. And below -40 SAT I have never seen any ice accretion.
But even if true... would you not use wing-anti-ice if your wings were full of ice???!!
P77