If multimethods were a subset of static typing, then regular methods would be too. Methods are dynamic by definition: the type of the method's target is inspected at runtime and used to dispatch to the correct function entry. In Python, it's explicitly stated because the first argument of all object methods is "self". Multimethods extend this to do dynamic dispatch on all argument types, not just the target object.
Most Python object methods are declared at compile time; this doesn't make method declaration a "subset of static typing". You can add new methods to classes and objects at runtime using Python's reflection and introspection; you can just as easily add new multimethods at runtime using this framework.
Finally, the @decorator(...) syntax at compile time is just syntactic sugar for something like the following:
def _foo(self, arg):
...
foo = decorator()(_foo)
So you can use decorators at runtime whenever you want too.
