> The twist is that each feed is encrypted in a way that only you can decrypt

Not true, the "content key" is common to all viewers of all posts, from a particular author. (hence the need to re-encrypt the world when you unfollow someone...)

The content key is common, like the PGP session key is common. But to obtain the content key, you need to first decrypt it by your private key. The content key is encrypted by the public keys of every intended reader, so each can have a secure copy of the content key. Again, exactly like PGP works.