I thought Onlyoffice supported both formats, but apparently you're right; OOXML gets native support and ODF is treated by the developers as an import/export process (based on the comments there).

To the end user, that makes it look like OOXML is the default and ODF is an inconvenient second option you have to manually choose, but the issues probably run much deeper than that.

Thing is, this is supposed to replace MS365 not Libreoffice.
And the collaborative work environment will receive docs from workplaces that maybe use MS365.
If those don't render correctly, the user will complain about jumbled docs that make no sense because the one image moved the whole document by 10 pages

And riddled with Russian spyware, but that's the icing on the cake

Russian ownership of OPENOFFICE is why this Euro fork exists.

edit: meant ONLYOFFICE. too many Os out there

Euro-Office and OnlyOffice don't "only support" OOXML. Where did you get that idea?

And not every software developed in Russia is "riddled with Russian spyware." The code is open so where do you suppose all this supposed spyware is hiding? Or is that just fear-mongering?

I'd love for LibreOffice to do a serious modernization to be a viable alternative. But for most users, it feels like a downgrade whether or not it is technically better under the hood. I use OnlyOffice these days even though it has a lot more bugs because the UX is just that much better in spite of it.

The EU wants to not depend on US software.

The format is standardised. It might not be a standard you like, but it’s a standard that the US can’t “take away” from the EU. Microsoft, especially in cloud form, can remove access to Office tomorrow if they’re pressured to by the US government.

We might have a desire for better file formats. The EU doesn’t give a monkey about our crusade, it purely cares about not having US leverage hanging over them like a sword. And the underlying file formats, exactly because it’s a ISO standard (however poor it may be) is fine for this purpose.