The fact that W3C defines the protocol doesn’t stop large companies from doing whatever they want. Have a look at Google: their web browser has become so widely adopted that Google effectively controls what is considered part of the spec, not W3C.

If Meta’s platform grows to become the biggest fediverse project, they will control the spec and others will either have to follow, or risk dropping out. This is just like how Firefox is forced to follow Google to ensure all websites work properly on Firefox, even if these sites don’t comply with the spec.