Yes, take a look at those method signatures. They are imprecise and incomplete. And that is for MRI itself. Very few Ruby projects has documentation anywhere near what MRI does, whether separate or in the comments.

But even if it did: Rubyists are far more likely to be tolerant about documenting type information in optional comments than including them in the code.

I think there's room for experimenting with Ruby's typing, but I also think the most that project can hope to achieve would be extensions that could see some limited use in the odd little piece of performance critical code. It's best hope of success, actually, would be that JRuby and other alternative implementations grow more popular and make it harder for people to rely on C-extensions.