On the other hand, there are no fans pushing dust inside that PC(in fact, the whole inside of this thing can be sealed airtight) and the amount of dust that naturally settles on the radiators outside should be much smaller than on any active-cooled PC.

These (AirTop, Fit-PC, etc.) are completely sealed machines so no dust enters. It does build up on the case - depending on your environment - but doesn't effect the thermal characteristics at all.

It looks like the airtop has open air pipes that work like chimneys, letting the heated air inside flow up and out of it. These ought to be easy to clean with compressed air though.