(1) Because of computer problems (and my being a computer illiterate), I cannot press the "thank you" button to join the two other members who have thanked you. So I thank you VERY much in this post.
(2) I, too, think that the appositive idea is very interesting, but I agree with you that somehow I find it hard to accept. With you, I prefer the ellipsis explanation.
(3) I should point out, however, that in Professor Quirk's A Comprehensive Grammar of the English Language (which SOME people feel is the most
comprehensive grammar currently available), he writes this:
Reformulation can also be negative, ie the modifying APPOSITIVE (my emphasis) is NOT (my emphasis) a synonymous expression:
You should have consulted an ophthalmologist, not (that is) an optician.
(4) Nevertheless, I believe that most "authorities," pace Professor Quirk, still hold to the ellipsis theory.