I did some testing on 220.127.116.11 today. Beyond the unexpected definition of "Linked," I'm also noticing that my project filesize only increases if I add, then remove, an image in a Picture Frame.
For example, in my case, I just added, then removed (by overriding a master), then re-added the exact same 50MB image multiple times to a page. The filesize of the project increase by about 50MB each time.
I expect that, unless I've opted to Save document history, deleting an image from my document would its embedded date from the project file.
Additionally, even with Publisher's definition of "linked" files, it should still be possible to preserve space by storing a single instance of an embedded image within the project file if that embedded image is used multiple times across a document.