AlanF said:
Firstly, why do you say dust is flatter than sand? Dust particles are usually modelled to be spherical or cylindrical in scientific studies, ie the same shape as sand and certainly not flatter.
How dust is modeled in scientific studies isn't really relevant to it's ability to penetrate a 'dust-resistant seal'. Neither dust nor sand are actually spherical or cylindrical, but in general dust is smaller and has a higher surface to volume ratio than sand. Here's some household dust:
Some of it could certainly be described as 'flatter than sand'.