I checked the official website of #OpenGraph (see: https://ogp.me) and I don't see the fediverse: namespace anywhere. Which means this fediverse: namespace is not an OpenGraph tag and will more likely not work without a proper prefix namespace, correct?
So, what am I missing here? People are already adding it. O_O
Update: Relevant thread/discussion about this fediverse: namespace.
As I mentioned elsewhere, there is a case for providing json-ld metadata - which is already in use and has been blessed by Google (not that I care) and dumping both OpenGraph and X/Cards in favour of open standards over proprietary and non-extensible solutions. The Alphabet currently only blesses @context values using schema.org, but this is where the fediverse can assert a standard context that works for us and just start using it. And it's already defined!.