I really like how readable the overview paper is [1]—I'd recommend reading it if you want to understand what is being proposed here. I especially thought the section on a passive operating mode was interesting. The idea that you can hide an ACK in another message, which is then broadcast to ACK both of the original messages is pretty clever. However, since Bradley's message to Charlie is also broadcast to everyone, wouldn't Eve get the same amount of information as if Bradley had just ack'd the message in the first place?
Anyhow, assuming I misunderstood something, passive listening doesn't seem to work well with their description of stream scalability, since Alice would now think she knows Bradley's stream, when it was actually Charlie's stream. For this to work, I suppose Alice would have to know that Bradley would be a secret listener to begin with.
In any case, it is definitely an interesting read. Seems like the type of thing Julian Assange would love to have. [2]