AFAIK the editor (shell) is WPF (C#?) but the internals are still mostly C++/Win32.

The shell still has a lot of C++ in it, yes (but also a lot of C#).

But the shell is a relatively small part of the entire IDE - the bulk of it is the extensions that actually provide support for various languages and technologies. Those also have some C++, either as legacy code, or because the team just preferred it, but by now C# is definitely the majority of it, and it keeps trending in that direction.

A lot of it can be very non-obvious and defying common sense. For example, if you take VS 2013 (pre-Roslyn), the C++ project system was written entirely in C#, while the C# project system was written mostly in C++.

Even in the .NET Core world, the managed runtime is written in C++, the native runtime (for .NET Native) is written in C#.

Seems plausible, I don't know enough details to argue this.