Please do not email the police to ask for legal advice or try to get them to admit that open carry is legal in Washington or any state. Frankly, it's best to ignore the police unless you need their help, or are making a complaint against them either in court or thru their chain of command/political controllers.
Nobody is likely to ever getmany or most police forces to go on record that open carry in a holster is legal - the only way to work the issue is to open carry in public such that they face a fait accompli.
Ignore the police and just go about your business in a manner obviously notintended to alarm anyone.
Yes, the WA state statute is a little vague, but so are many laws, and the recent appeals court case seemed to limit it's application to at least slightly unusual circumstances - i.e., Mexican carry where the hammer is cocked back. Applying the statute to all holstered carry would lead to an absurd result
The cocked back hammer probably does cause alarm in some people. The Mexican carry thing does look awfully bizarre. Taken together in an urban setting where even open carry in holsters is rare, and well, there you have it. That is probably the limit of the law's application to the open carriage of a handgun in public.
Now walk thru a mall with a shoulders AK-47 clone and spare mags, well, that might also alarm folks within the meaning of that statute, not to mention the obvious problem with muzzle control of a long gun in a crowded multi-story building