Not sure this is the most performant way to do it, but the project itself sure is impressive.
LibreOffice Online does not use Broadway. The server side uses LibreOfficeKit and the client side tile rendering is based on http://leafletjs.com/ (heavily modified).
Mh, got it from the parents wiki link and thought it was interesting that they chose something like this.
You are right, that wiki page is confusing. I'll ping devs about it later.