What does this even mean? The attackers had the users' credentials, by resetting them. They had all the access the users had. There is no kind of encryption that would prevent them reading the messages in this situation.

Twitter is fundamentally a web app. Users can log in from any browser and read their messages, which are stored on the server. This is a very different situation from Signal or WhatsApp, where an account is tied to a device, and messages can be stored there.