When I say bidirectional disconnection I'm talking only about directly between the defederated instances in question since some people are suggesting that the instances are still directly sharing content in one direction. In your example the instances are still bidirectionally cut off, you're just suggesting that a third instance could relay content between them. That would be theoretically possible but I don't think it works like that currently, because that idea gets shot down in Mastodon for several reasons, including the fact that trusting content relayed to you from an intermediate server could open up the possibility of the relaying instance tampering or impersonating user accounts or faking content.