As far as I can google, both of those states have had a substantial sales tax for many decades — Washington's is one of the highest in the country!
What am I missing?
Think the article confuses sales and income/capital gains tax. Being in a small market was an advantage from a sales tax perspective. Being in WA or NV was advantage from income/capital gains tax perspective. The latter gave Amazon an advantage in hiring. And, later when successful, allowed Jeff and early employees to realize more wealth from their stock/options.
Nevada, on both fronts, would have been as much of more efficient. But, I’m guessing, the difficulty of recruiting great technical talent (which was needed in the early days of Internet ecommerce) would have swamped those advantages.
For much of Amazon's early history, the sales tax in Washington was only relevant if you happened to live in Washington. The set of areas where Amazon must collect taxes has grown, and at this point for all I know might be everywhere in the US.