No, A doesn’t get the content from C, but if C is not defederated from A, yes, C gets the posts from A (feed). It’s called asymmetric defederation. For example, that’s the case with sh.itjust.works and beehaw.org. Those of us that subscribed to communities on beehaw.org before it defederated from us, we still get posts from beehaw in our feed, but beehaw doesn’t get any from us. And if we actually comment on the post, that comment will be visible only to members of sh.itjust.works, no one else in the federation, because the sync feed from beehaw takes presence over ours, so the instances that can talk to beehaw, get the sync feed from beehaw first (or if it arrives second, ours is discarded, because beehaw is the original instance on which the post was made).

Just also want to add:

When A federates with B, it does not get B’s content. A only gets content from B that A’s users subscribe to.

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