The comparison to ".d.ts" seems apt since both are languages/grammars focused solely on declaring types. How they are interpreted is a different matter of cours

The comparison to ".d.ts" seems apt since both are languages/grammars focused solely on declaring types. How they are interpreted is a different matter of course. TypeScript assumes they are strict declarations of the JavaScript code, while Steep/Sorbet will use the declarations to type check your code. In your example you would get a type checker error.