Technically, using wd on the actual metal cable core and plastic outer is actually no problem 3D, (plastic on metal), its all the other areas , gears and slides that are plastic to plastic that are of concern, its difficult to "only" spray the cable core, and not get it over the other stuff (plus it can run down where you dont want it)
The other problem is , that 99% of the time, the sticking is caused by the plasic to plastic slide parts on the side of the vent/heater core chamber anyway, not actually the cables , so its much better to use a product that it safe on "everything" under there.