It's there on the old MBPs too and annoys the hell out of me. I even tried vaccuuming the speaker grilles 😀. It helped a bit.
for some reason im thinking that this 'dust' isnt from dust
maybe its the way the holes got cut which is creating that weird reflection. plus, most of us has not had our laptops for very long so im pretty sure the dust wouldnt have gotten on that fast