Okay. But this method doesn’t address that the service doesn’t need the message to include the sender to know who the sender is. The sender ('s unique device) can with 100% accuracy be appended to the message by the server after it’s received. Even if we trust them on the parts that require trust, the setup as described by the blog doesn’t do anything to prevent social graphs from being derived, since the sender is identified at the start of every conversation.

If we trust them not to store any logs (unverifiable), then this method means they can’t precisely know how long a conversation was or how many messages were exchanged. But you can still know precisely when and how many messages both participants received, there’s just a chance that they’re talking to multiple people. Though if we’re trusting them not to store logs (unverifiable), then there shouldn’t be any data to cross reference to begin with. So if we can’t trust them, then why are we trusting them not to take note of the sender?

The upside is that if the message is leaked to a third-party, there’s less info in it now. I’m ignoring the Github link, not because I don’t appreciate you finding it, but because I take the blog-post to be the mission statement for the code, and the blog doesn’t promise a system that comprehensively hides the sender’s identity. I trust their code to do what is described.