I believe zacharycohn meant "regressive" as in "imposes a harsher burden on lower-income households than on households with higher incomes."[0] In this sense, Washington state does have a regressive tax system, in part thanks to 0% income tax and higher than average sales tax.
Yes, but I think it's a relatively new or transient interpretation. I couldn't find much of an authoritative basis for it. It would be great if you could post any links that you know of.