""Leverage your C# experience to feel right at home
If you're an experienced C# user, Godot offers you first-class support for the .NET platform. Power your game with familiar libraries and give them performance boost, while still benefiting from close engine integration.
Note: .NET support is provided as a dedicated engine executable. C# support is available for desktop and mobile platforms as of Godot 4.2. Web support should be added in the future, but until then, Godot 3 remains a supported option.""
He's talking about the Switch port library. It didn't support C# yet.
But also on the wider topic Godot still struggles with making C# a first class citizen compared to its home baked script language. They're improving things but it's not great yet.
No C# or GDNative/GDExtension: Only GDScript is supported. Native extensions are not supported, but you can try to convert them into internal modules.