Going to need a source for that claim - generally the largest industry in a given area is government, then hospitals or schools, then some specific niche. I don't think there's any airplane manufacture in NorCal anyway - this would be a bit more believable about Wichita.
Because you're looking for "airplane manufacturer" and possibly large one. Though for example just a stolen throw from Google Mountain View campus you have Northrop and Lockheed-Martin.
I can't find the original webpage after 7 years, but the source was San Mateo County page related to county-owned airports, and it included all sorts of aviation industry (both flying and ground side)
