Seattle and the Puget Sound is a beautiful place but horrible place to live. They have an absolutely useless government that has no idea how to solve any of their problems. So they end up with stuff like this.
The study results show that there was probably another case (or multiple) introduced around February 13, meaning that there had only been about two weeks of community transmission instead of the six that officials thought. But because they thought it had been spreading undetected for six weeks, they reacted very strongly and put in strict measures quickly.
So it was probably technically an "overreaction", but that's what made it so effective and kept the situation from getting as bad there as NYC and other places.
Here's an Ars Technica article about it: https://arstechnica.com/science/2020/05/washingtons-covid-19...
And here's the actual study, which also includes a similar situation in early Europe cases: https://www.biorxiv.org/content/10.1101/2020.05.21.109322v1