And here's the reply:
cover:false filters on whether there is a cover for the book in calibre. That is not going to change. Whether your book file has a cover or not is immaterial.
In other words, I'm stuck right where I was, 3389 books to manually manage in order to find the ones with no cover art.
Either re-downloading the cover of copying the existing cover (within calibre) and pasting it works to update the cover:false search. This is a pain to do with so many books.