Interestingly, that could have happened. Not only did LO make the effort of rebasing all their changes on top of a particular AOO release, they also were set up to incorporate all the new work from AOO into their branch. For instance, they quickly incorporated the sidebar code. Even today, one of the LO developers still goes over each AOO change, merging any worthwhile ones.
What actually happened? For some reason, development on AOO stalled. What little work still happens on it, is almost always replicating work LO already did two, three, sometimes five years ago. See it for yourself at https://cgit.freedesktop.org/libreoffice/core/log/?h=aoo/tru... : pick a commit, and look for the "Notes" field. For instance, picking "Port main/fileaccess to gbuild" I see the note "prefer: 1f9bc2b2aa8168f9c164044058b117d2a17d83ad"; that git hash is for a LO commit from 2011 (https://cgit.freedesktop.org/libreoffice/core/commit/?h=aoo/...).
In the meantime, LO did a lot of cleanup work. Finishing the build system move to gbuild, cleaning warnings across all the code base, removing plenty of obsolete or unused code, running several static checkers, and more. As I noted at https://lwn.net/Articles/699108/, the recent AOO CVE was probably found and fixed on LO two years ago, as a result of running a fuzzer together with valgrind. If you wanted to pick an "upstream project" to base an office suite on today, it makes more sense to pick LO instead of AOO.