Important bit of information is further down in the article: OnlyOffice is Russian. I would therefore view any collaboration as a risk. It's not adequate for strategic reasons as well as sovereignty.

If you need Docx compatibility to interface with the rest of the world, are you better off with the at-least open source option or the sign-your-life-in-eula-and-O365-subscription option.

This isn't rhetorical. I don't know which is worse. I lean disliking Microsoft more, because jazz hands at Windows11, and OnlyOffice at least runs on Linux, but it's still not a fun position to be in.

LibreOffice and other alts definitely don't have as good of Docx compat.

Not sure why you were downvoted; it's quite possible that Nextcloud was more concerned about the political independence/potential sanctions aspect.