The amount of dust on the filters was daft, there was a fair bit inside as well, but it has a thin foam? at the front, behind the metal grill, and a nylon mesh at the bottom. Most of what seemed to be in his case was very fine particulate stuff, which nothing is going to stop. The amount on the front suggests a dusty environment, or at least him not cleaning for a very long time.
I'd say it was more on him, than on the meshify (I got one at the start of this month, so can only tell you about the filters, not the dust buildup.
Another possibility is that his 140mm fans weren't upto the job of drawing air in enough to get positive air pressure, and so dust could simply fall in the case (there's a mesh cover on the top), but I'd guess if he's gone to the effort of replacing the fans, he's probably got fairly good ones in there.