That's not how I understood defederation. If an instance defederates from you, that instance stops seeing stuff from your instance. But not necessarily the other way around, as defederation is a one-way action.
So if the Cow instance defederates from the Poopie instance, people from the Poopie instance can still see content and comments from Cow users. But Cow users cannot see content or comments from Poopie users. For the scenario you're describing to take place, the Poopie instance would also need to defederate from the Cow instance.
That said, it's still not quite shadowbanning. The admins of the defederated Poopie instance would be aware that Cows were not seeing their content. It would depend on the admins to inform the Poopie users that they've been defederated. If the users were not aware of the defederation, then it'd effectively be a shadowban.