> because all the interface stuff comes from statically typed languages.

No, it doesn't. It comes from theory that came after the languages.

> Some* dynamic languages borrowed it, but most use "implicit" interfaces

An implicit interface is an interface, and is exactly the sort of thing I'm talking about in GP. The point is that you think about the object in terms of its capabilities, rather than some proven-up-front categorization that it fits into. What it does, not what it is.