> Suppose Bob's codebase has a structure implA that implements interface A, and you want to use in terms of its interface. If the language requires Bob to declare implA as an implementation of A, you have to persuade him to do so in his codebase.

Not necessarily, in some lanugages you can add an interface to type even if the type is foreign and you only control the interface. I believe this can be done in Swift, Rust, F# and quite likely some others...