That is someone's port of Electron to .NET, the official Electron uses JS (and Chrome). Electron is open source, but was created by GitHub, which Microsoft bought.
Ahh ok, thank you for clarifying that, I was rather confused about it all. I didn't know much about the project by I managed to invent a wrong memory that it was always a C# thing, not that insanely bonkers js, node, applications are webpages thing :)
https://en.wikipedia.org/wiki/Electron_(software_framework)