That's quite contrary to the whole CCC philosophy though of freely sharing knowledge. But indeed he linked to it himself.

One thing I don't like about Signal though, is that he forbids third-party clients. It's a bit contrary to owning your information, which is one of the points he makes in the talk. One of the reasons I want this is to integrate all my chats into one single app - there's too many apps right now screaming for attention. That's why I'm more a fan of Matrix, which is truly open.

He linked to the video himself in the Looking back as the world moves forward (https://signal.org/blog/looking-back-as-the-world-moves-forw...) article.