Just to be clear, I intentionally never said anything about static types. The type system is "strong" in the sense that variables never change their type or are

Just to be clear, I intentionally never said anything about static types. The type system is "strong" in the sense that variables never change their type or are "interpreted" based on context, they don't even change their value after assignment. And whether your types are runtime-checked or statically checked doesn't really matter with regard to the invariants that hold for your code. In the former case, you'll have to be able to handle type errors at runtime as well, which is reasonably easy in Erlang due to the strong isolation it provides.