Maybe that was a misunderstanding: I didn't mean to say that interfaces should exist purely in the head of the programmer. I did say that implicit interfaces/protocols (Go, Objective-C) are better than interfaces that need shared dependencies (Java/pure virtuals in C++). If I had to order it I'd say: duck typing better than explicit interfaces; implicit interfaces that are backed by static type checks "better than" duck typing. The "better than" in quotes for the latter because it really is more about how much static analysis the language supports and it's more a matter of opinion.

But I'm relatively convinced that interfaces/protocols that require coupling of implementations are clearly inferior to the ones that are implicit.