It's actually got absolutely nothing to do with City of Seattle government. This is an initiative by the King County (the county where Seattle is located) Prosecutor's Office and the Sheriff's Department. It does not (yet, hopefully) apply to Seattle Police Department or the Seattle City Prosecutor's Office. So while many of the affected youth may end up benefitting from the initiative if they land at the KC Juvenile Justice Center, if they're arrested by an SPD officer, they're likely to hear the same old Miranda.

(And another point of clarification 'cause I read it too and can't help myself: an opinion piece by George Will isn't exactly an "editorial." I'd seriously doubt that the WAPost editorial board would agree with many of his dubious positions in the column.)