I would predict/expect that both LibreOffice and MS Office (with their modern XML-based formats) are actually mmap(2)ing some temp file and treating it as an "on-disk working-state heap", and then importing from interchange formats by allocating from that heap / exporting to interchange formats by chasing pointers that end up inside that heap. (This is, after all, what every RDBMS does for its working state. It's pretty optimal.)

Even if you have a memory-mapped interchange format, I'd still expect them to have a separate disk-backed working heap for all the stuff that doesn't belong in the file but is nevertheless very large (e.g. cached intermediary computation results of spreadsheet cells); and, if they have it, they may as well just use it for most things by default. Thus, I would expect that even in old versions of MS Office, the in-memory data structures were actually an interchange format of sorts—not the ones being updated with each keystroke, but rather ones that'd be memcpy(2)ed into on export. (This also prevents you from either having to add a page-table structure to your file, or else constantly "defragment" it as data structures change.)