Yeah, I think that's what Garner is referring to — vaguely speaking, the "idea" in "person, place, thing, or idea." The other example he gives in section 2 is "all is well": "all" here is a collective abstraction referring to the entirety of existence and isn't really a
thing per se.
Like I said a couple of posts ago, I'm pretty sure the difference we're having over "All I Saw Was/Were Stars" is in which of those two categories of Garner's that we're viewing "all." I'm looking at it the first way: what I saw was a group of stars, which is a collective, physical thing, so I'd conjugate the verb as plural. Whereas you and Hellacia are looking at it in the second way, from a level of abstraction: what you saw was the entire field of vision and that entire field of vision was stars, so you'd conjugate the verb as singular because the totality of vision is a collective abstraction and not so much a physical thing.
I think that's kind of a roundabout way of looking at the imagery of the title, but I also can't really argue that it's invalid either.
