That's not true. If Outer has a member Inner, Outer always has to invoke `my_inner.foo()` to use Inner::foo, and `foo()` always refers to Outer::foo (and some languages will force you to write `self.foo()`, which is even better).
If Outer extends Inner, though, you can't tell whether `foo()` refers to Inner::foo or Outer::foo without checking to see whether Outer overrides foo or not. And the number of places you have to check scales linearly with the depth of the inheritance hierarchy.
If object A calls a method of object B (composition), then B cannot call back on B, and neither A nor B can override any behavior of the other (And this is the original core tenet of OO: being all about "message-passing").
Of course they can accept and pass other objects/functions are arguments, but that would be explicit and specific, without having to expose the whole state/impl to each other.
The same pinball of method calls happens at almost exactly the same way with composition.
You save some idiosyncrasies around the meaning of the object pointer, and that's all.