Because passing messages still means dispatch on concrete types - something concrete sends a message, something concrete receives a message. That’s the whole ballgame - get one concrete type to talk to another. Even if ‘concrete’ is in a seemingly typeless language, we can Hindley-Milner our way down to this.

I think it’s telling that Go, Rust, as well as more inspired stuff in later editions of C++/Java/C# all seem to be demonstrating that nominal inheritance-first polymorphism is not the only or always the best maze of abstraction for getting to a concrete dispatch from type A to type B in imperative-feeling languages.