ONLYOFFICE Workspace includes its own document storage and management system, that is why it cannot be connected in such way. ONLYOFFICE Docs is the product which does not have those, therefore you can connect it to Nextcloud as well as other document management systems.
Thanks a lot for clarifying this, yes I see the Workspace has its own document storage, the only reason we need Nextcloud is previewing some files like AI, STP, DWG etc, if Workspace could add this function in the future it will be great.
For now I have to install one more docker for Docs.