For a very long time .Net was Windows only. That only changed after Balmer was gone and Satya came in.
Some cross-platform considerations have been built into it from the start, though. And there has been an official open-source implementation on a non-Windows platform. Perhaps more to show that it's possible than as an actual usable artifact, but it was there.
I think describing Mono as Official is a bit of a reach. AFAIK they benefited from the patent promise but that was about it until the acquisition.
If you mean .NET Core as official open source, that's true, but it's a relatively recent thing.
There was Rotor on FreeBSD. Probably more a proof of concept, but it was there.
It's sad how so many articles and comments come off just pure PR now. Not saying your comment is, but I have to wonder how much of the "news", comments and internet content is now just PR.