The illusion that all designers are on Mac is probably 
  because HNers don't often meet the ordinary designers who 
  have a boring corporate job or are struggling freelancers 
  somewhere far away from San Francisco.

Out here in the midwest, every design freelancer I've known has had a Mac, without exception. Every small-biz graphic design studio or ad agency I've come into contact with has been a Mac shop.

3D artists, however, have been just as consistently Windows-based. I've never met a single one of them with a Mac.

That's why I said “most design studios” and not “all designers”.