Okay, let’s say I believe everything they say. The ballot is double-enveloped, there are no traces of who the voter is on the ballot, there is no corruption of how the ballots are handed out so that no one records the association of ballot ID with voter, etc… Let’s say all that is true.
How does King County claim to control what happens before the voter mails her ballot? How can we be sure no one was telling her how to vote, threatening her one way or another as she filled out her “secret ballot” at home? How can we even be sure who mailed the ballot?
Without some really fancy crypto tricks, Vote by Mail is inherently coercible. To claim otherwise is just denial of basic facts.