The Signal protocol is only responsible for encrypting and decrypting messages. Yeah, Keychat uses Signal protocol to encrypt / decrypt message.

Interesting, I just read about the double ratchet algorithm:

Signal Messenger

Signal Messenger: Speak Freely

Say "hello" to a different messaging experience. An unexpected focus on privacy, combined with all of the features you expect.

Does this mean that users could publish their nsec and their old DMs would not be accessible?