This should work if all your pages have a frontmatter attribute "authors" with "admin" in that list. If you don't, there's currently no way to do this because SilverBullet actually doesn't really know who authored what right now. With revisions this could indeed become available by inspecting version history I suppose, but that's not currently accessible via the object index. Is this what you're after?
I'm not asking for silverbullet to know who authored which and register in the object index; instead, I would like to manually write in the frontmatter something like
authors: admin pete
and be able to query for the pages who are authored by admin (or pete).
Specifically, the error returned by the query is Cannot use includes on a non-table or non-array value.
no, but to be honest I do have "hacks" in place for e.g. tags where I support both this "spaced out" version as well as lists, even with # in there, just for convenience. But that's specifically done for tags, not for everything.