You're missing the input type, essentially. Those are just array types. The TypeScript type signature more of a function type, it expresses flattening a n-dimensional array (input type) into a flat array (output type).

You're missing the specialisation of Object/Any. For example Array.flat called with [int, [bool, string]] returns a type [int, bool, string]. Admittedly this is somewhat niche, but most other languages can't express this - the type information gets erased.