Why would it be less meaningful to say types are checked at runtime with ducktyping? The nature of ducktyping is that the specific class of an object does not matter relative to behaviour, but a class is not entirely equivalent to a type.

If I need an object that implements method `foo`, and don't care about class, then "objects that implements foo" is in itself a type, that can potentially be inferred and checked be it at runtime or before.

> The majority of functions written in Python, even the ones that have type annotations, do not effectively have "assert isinstance(...)" in the program text below their signature, which is what I'd expect after reading "check types at runtime".

You're thinking his "checks types" too narrowly. Every time I try to call a method on an object in a strongly typed language, typing is involved. It doesn't so much "check" it as look up the method to see whether this method applies to this specific object at this point in time and decide whether or not to throw exceptions.

But the point remains that it is a typed. And strongly so - in both Ruby and Python objects has a type associated with the object itself, unlike e.g. C or C++ which are weakly typed because it is the variables that are typed, not the values.