I don't see why anyone would want runtime type checking the way AtScript does it.

With static typing, the compiler can catch type errors early, at the cost of rejecting some otherwise valid programs.

With dynamic typing, that type checking is delayed until execution time. More programs are valid, but type errors are caught late.

With AtScript-style late type checking, they took the dynamic type system and introduced more type errors. This decreases the amount of valid programs, yet catches no errors at compile time. It does nothing but increase the amount of bugs.