Not sure why you think it PNW only. The number came from the CEO of AMEX, back when they were the exclusive credit card provider for Costco. He said, in 2015, that the average Costco card holder made $100K a year. So presumably that would be in all of the US.
More recent attempts at analysis put the number closer to $125K these days for all their customers worldwide.
Which makes sense, given inflation and the fact that they only build Costcos in more affluent areas and target business owners.