I'm going to speculate that Airbus may be more "relaxed" about turning on WAI with slats deployed not because the system works less well - there is no reason why slat deployment should materially affect the performance of the system - but rather because the aerodynamic effects of ice with slats deployed was found more tolerable than for the slats retracted case.
It's not that it's less effective - but rather less required. As a guess.