Where is the "administrative boundary" of any "unincorporated community" defined?  (I mean, strictly?)

If you say it is a (US Department of Commerce) Census Boundary, that's OK, but those are tagged with boundary=census.

By wide OSM community consensus, unincorporated areas (in California and at least 47 other states), do not receive an admin_level value, of 9 or any other value on the admin_level key.

A boundary=administrative + admin_level=9 tag is (sometimes, under very recently agreed to by local, New England area residents) occasionally tagged on unincorporated areas in New Hampshire, Maine and (more rarely) Vermont, but it was a real stretch to get there, though OSM did.  See our United States/admin_level wiki page (and its companion, United States/Boundaries).