the link you refer to talks about collabora and allotropia. the first is what powers Nextcloud Office/Collabora Office online and mobile apps (using coolwsd). the latter is allotropia, who is behind Zeta that is mentioned here. both have a slightly different approach to how this works. as i understand it, all of this is upstream and not really frozen; perhaps that page needs an update?

Note: "LibreOffice Online is not intended as a standalone software. His goal is, instead, to be integrated with other tools to edit their documents." ... while Zeta and Collabora are standalone.