Beyond marketing fluff, I don't think anybody at Microsoft genuinely believes they have an "open office format" or an actual "standardization". Even Apple back in the day had to reverse-engineer the Microsoft formats. [1]
Whether you'd like to denounce OnlyOffice taking part in this masquerade or not is a political issue. But giving Microsoft any form of benefit of the doubt on this matter is historically wrong and, I believe, ethically evil.
[0] https://news.ycombinator.com/item?id=45144758
[1] https://www.eff.org/deeplinks/2019/06/adversarial-interopera...