Nice conversation there. But GPT has answered your question by himself:
> Why C# makes sense for this author, though
The README basically answers part of it. ArcadeMaker has been developed in stages over several years. It originally let game authors write C#, then the author independently created Exp, then returned to ArcadeMaker and replaced the C# scripting system with Exp. The author explicitly says the language itself is part of the project and that much of this was built for the challenge of writing it personally.
> So Exp isn't merely an engineering dependency they failed to replace with Lua. Building Exp is one of the hobbies.
And another important reason - i don't know rust, and i do know (and really love) c#