I think if it was a distro like VSCodium, you could call it a fork. But Monaco is a small piece of VS Code. It's a text widget. A _nice_ text widget, but you can play with bare Monaco here:

https://microsoft.github.io/monaco-editor/playground.html

You could shove this widget alone into electron and call the result a text editor, and I mean, Notepad was basically this for the windows text entry component for years, but it's a long way away from VS Code, or from Theia.