When a large company takes an open protocol, embraces it using adding users to the network through heir platform, then extends it using proprietary means, they have full control over how the protocol runs in the network.
When the open standards are forced to make changes to be functional with the dominant proprietary app that is poorly (and sometimes incorrectly) documented, open source groups are constantly on the backfoot in order to maintain compatibility, and that makes it harder to compete on their own right.
A second example given is LibreOffice, whose documents are made to fit the XML standard by Microsoft, but there are quirks in their documented standard that if you follow it too closely it isn't formatted quite the same as the document produced in Microsoft Office, so they were pressured to effectively copy MS and deviate from the standards MS claims to follow.