Since they're looking for a result, even if that constitutional provision said "weapons" and not "arms" it likely wouldn't matter. They'd probably then rule that since no threat was imminent, the defendant wasn't bearing in self-defense and so the constitutional provision still doesn't apply. That would gut it entirely.
Not the first time the WA SC used suspect definitions to obtain a result. As I recall they're also rather aggressive at denying any and all attempts by the voters to restrain the legislature's taxing authority, basically by defining all tax initiatives as "multi-issue" and then striking them down because ballot initiatives must be "single-issue".