.Net has been (_almost_) entirely open source for years now. The whole closed source and Windows-bound .Net Framework is a legacy thing of the past now, since .Net Core (which is now named .Net) everything but the debugger is open source. It's all on GitHub. You can do all your C# development on Linux nowadays, too, with VSCode or Rider.
I think he meant that open source third party add-on packages for .NET were basically non-existent. Not .NET itself. Your "_almost_" qualification has me concerned. Over the decades I've almost been fooled many times by Microsoft's claims of being open and/or interoperable only to find out that it's "almost".
