There are plenty of pretty affluent households in Seattle, but it's not quite 20% of them that make >$250k. What the linked article says is that the average (mean) income of all 60,000 households in the top quintile is $250k, not that $250k is the lower threshold for entering the top quintile.

I can't find detailed enough information to say what percentile a $250k household income in Seattle actually is. What I can find, from the 2015 census estimates, is that $200k is 87th percentile [1].

[1] As aggregated here: http://www.city-data.com/income/income-Seattle-Washington.ht...