You cant design an airfoil to not accumulate ice - if its stuck out in the breeze, it accumulates ice.
What that implies is that they have designed the wing so that ice on the rest of the slats/wing doesn't degrade the handling or performance enough that they need to antiice anything except the $4 slat. Usually there is a critical part of any wing - if you can protect the critical part, the rest can be left to fend for itself. The question is, how much do I have to preotect - it can be "all but a little bit", or it can be " just a little bit". Depends on the rest of the design.