This makes much more sense to me:
Well, the airlow travelling along the back face of a foil has a tendacy to detach itself at the thickest part of the section - limiting the maximum downforce for a given foil profile and AoA. However, if you introduce a thin layer of turbulance at that thickest part of section the flow will stay "attached" for much longer increasing the max downforce for the same profile and AoA.
So in a nutshell McLaren can run less wing (less drag) yet still get decent levels of downforce.