I don't think .NET Framework is a supper set of all other implementions of .NET. For example, Mono's SIMD types are not in .NET Framework. It is even possible for types to exist in .NET Core that don't exist in .NET Framework.

I think you could say the following:

    .NET ECMA Spec ⊆ .NET Standard
    .NET Standard ⊆ .NET Core
    .NET Standard ⊆ Mono
    .NET Standard ⊆ Xamarin
    .NET Standard ⊆ .NET Framework

You're confusing intersection and union.

Oh, derp, you're right.

Still, .NET Framework isn't the superset of everything. As I said, I find your description not useful because it simply doesn't capture how the .NET stacks are designed.