I’d say that building=commercial is definitely a mistake and should be removed - it was part of that user’s first few edits. The only thing I can’t see is whether that relation was ever tagged as the Sonoma boundary (if that is really the city boundary).
I’d say to go ahead and remove the building=commercial and see if the boundary should be identified as part or all of the city of Sonoma; the multipolygon doesn’t seem to be serving any function otherwise.
Possibly the easiest way is to drop them a message via the changeset discussion asking (politely) what they were trying to do? Maybe the were trying to tag a single commercial area in Sonoma - if so, we’d need to find out which to finish off what they were trying to do, if it wasn’t one of the two buildings they’ve also added in the same changeset.
I’d definitely try and “be gentle” - OSM can be pretty daunting to newcomers and I suspect that the US (even that bit of CA) needs all the real “on-the-ground” mappers that it can get.