Are you sure that's not just a VNC-based demo of the desktop version of Collabora Office?

I wouldn't be too surprised if their web version was like that (given how long ago the first version came out), except Nextcloud uses a Collabora-based system for its online editing, and I doubt they'd go with something like that.

There are online ODT editors (e.g. https://webodf.org/demo/ci/wodotexteditor-0.5.9/localeditor....), so even if Collabora's is bad, it wouldn't be too hard to make a better one.