Unless we ever get the full archive of Microsoft emails, meeting minutes and recordings from all the secret microphones they didn't have in their meeting rooms, I don't think you can ever disprove this claim. It's generally impossible to conclusively disprove conspiracy theories, because you could always claim you're only showing there are no documents proving the conspiracy, but there are no documents disproving it.

The author is just implicitly appealing to Occam's razor here, as people often in face of accusations of a plot. They can show that Microsoft has backed the ANSI accreditation of ODF[1] and eventually implemented support for ODF import and export in Office, but that's not enough to prove there was no conspiracy.

Instead, the article just provides a very plausible explanation for the complexity in OOXML. Does this explanation thoroughly disprove the accusations of a plot? Clear not. Is it more plausible than a great plot to crush a bunch of competitors that had no market share and kill a better standard document format that Microsoft did end up implementing in Office? Yes. This is probably as far as we can get.

[1] https://news.microsoft.com/source/2007/05/16/microsoft-votes...

Both things can be true. It had a genuine purpose, but the fact that Microsoft will go out of its way to not implement anything better and less temperamental is an indication it's not really open. There's plenty of evidence of Microsoft dragging their feet at playing nice with the rest of the office ecosystem.

I'm not saying they shouldn't do that as a company maximizing shareholder value. But we should all collectively groan every time the topic comes up, not applaud them.