The App named Nextcloud Office has the app-id richdocuments
(you can find out with nc-apps)
So “disabling richdocuments” is done with occ this way:
occ app:disable richdocuments
But as you wrote:
but an app called “office add on” does not exist. There is
So you should first get a precise overview of which app you want to install exactly, because the installation is not that profane, some backends may have to be installed