There's two different scenarios, a community not hosted on beehaw:
they wouldn't see the chain at all.
On a community hosted on beehaw:
Message a would only exist on world's local copy of the thread, nobody not on world can see it. (This local copy is also not getting updated by anyone not on world.)
Basically, the true thread exists on the instance that hosts the community. Defederation cuts off communication with one server. If it's the server that hosts it, it's a lot more severe.