You’re correct. But most people who are using the plugin (at least most I’ve seen using the plugin) are using it primarily for the displays that can be built in other notes using dataview. e.g. Keeping track of to-be-read books, etc. There are some benefits to that, but I don’t think I want hundreds of books in my vault that I haven’t read, so I’m only interested in what it generates directly. Which, as you noted, is fine for a new book, but awkward if I’m trying to update existing ones.
What I’ve been doing, which is kind of a pain, but works, is to use the plugin to generate the file, Ctrl-X to cut all the YAML information, etc., delete that note, navigate back to my existing note, and Ctrl-V to paste all the generated data into the existing note. This is giving me new information – like publication data and page counts – that I didn’t have in most of my book notes already.