You're both right. Yes, it's WPF, and there are lots of parts written in C#. But the parent is right -- Visual Studio is C++ using COM and Win32 at its core, going back all the way to the Visual Basic IDE that shipped as part of Visual Studio 6. It would be a very difficult port to Mac, Linux, etc. As of this writing there is even no 64-bit version for Windows (although that's just a matter of spending the time and money to do it.)

The brand name has been re-used for other software. Visual Studio for Mac is Xamarin. Visual Studio Code is on the Electron platform.