Their point about OOXML has traditionally been that the format that Microsoft Office itself produces has never once matched the standardized standard they ratified. So Microsoft used it to check a box on some requirements sheet and muddy the waters (like this), but anyone actually following the standard would not have achieved actual cross compatibility with the massive gorilla in the space. But because it's "Microsoft's format" any issues would have felt like bugs in LibreOffice rather than bugs in Microsoft Office. In contrast the standardized ODF actually matches the ODF you find in practice.

That all having been said, I stopped paying attention to that whole scene a while ago, so I don't know what the current situation is, or if that still applies. It's possible later version of MSOffice actually moved to the standard version at some point, or that the standard was updated to match what MSOffice actually reads and writes. Possible, but I just don't know.