Microsoft Office has a project leadership that believes that Office Open XML is the best and most open office format. That's very convenient for them, considering that Office Open XML is a standardisation of the native file format of that lineage of office suites.
Now, OnlyOffice is presumably something written from scratch, unrelated to those two lineages. They chose to prioritise compatibility with the market leader's standard, and the second place in the market is upset that a competitor isn't favouring them instead.
I think this is a bit silly.