While the annual income given in the article is high, one can easily find average net worths in excess of this figure outside of these zips. These folks are apparently spending their wealth for today.
But this is the average, including every single household. No matter how rich the area, there are always a few people who are broke living there.
And in these rich areas, you're much more likely to find people with large negative net worths.
My wife is a CPA, and finds it not uncommon to have clients with a multiple of our annual income, and a fraction of our net worth. It's really easy to do. New S-class Merc every couple of years, multi-thousand dollar clothes shopping sprees 2-3 times a year, live-in nannies, huge houses requiring lots of maintenance and utilities, European vacations, etc.