Have you ever worked with a language that has header files (C/C++) or a language that can use them optionally (Ocaml)? In practice, keeping the files in sync isn’t difficult. In fact, it ends up being better (for me) in terms of readability, because I can look up the type definitions in one place, store them as context, and then read code that isn’t littered with type annotations. Type annotations add quite a bit of noise to code. I think that’s what Matz is going for here. You need to be able to keep the readability of Ruby, which he’s dedicated his life to.
Not to anecdote too hard, but the practice of doing type signatures out-of-line that you are describing is my absolute least favorite part of OCaml, which is otherwise a very lovely language. As for C++, it has enough other stuff going on that I probably can't say the header files are my least favorite feature, but they certainly don't make life easy. I think it makes sense to challenge that decision from an ergonomics standpoint, even if it makes sense within the constraints the Ruby core team has decided upon.
I very much like Ocaml signature files. Jane street recommends using them, as an example. There’s no other way to communicate the high level contract of a module. With type signatures that are in line with the code, the high level contract gets lost.
