Grey Area on Nostr: No, the recipient would also generates an ephemeral key, and uses that when replying. ...

No, the recipient would also generates an ephemeral key, and uses that when replying.

So it's always a one shot ephemeral (sender)/static (recipient) HPKE exchange, with sender authenticity guaranteed by the signature. Could also use the shared secret from the original post but force a different IV, but "replies are just the sender and receiver being flipped, code path is the same" is easier.