Thanks a bunch for your response. I guess an end user could always filter messages themselves, but it seems untenable at large scales.

This is not true. Followers are established by connecting to bittorrent swarms. From the paper: "The last network is a collection of possibly disjoint “swarms” of followers, based on the Bittorrent protocol, which can be used for efficient near-instant notification delivery to many users."

Furthermore, not all clients need to store and reseed all messages they receive. More seeders is obviously better for network health, but it's not always a reasonable option. The paper suggests that clients can choose to be "achivists" which means that they keep messages and seed them to others (so it's optional). Clients like mobile phones could easily disable this behavior.

Also Twister can't use Namecoin because the incentives are wrong. In Namecoin, miners get to create domains. It would be horrible if only Twister miners could make accounts. Instead they get to make promoted posts, so it has to be a separate implementation.