That's an interesting point too. They talk about not being a blob store, not wanting the serialisation cycle to hamper performance but makes you wonder how exactly they're storing the data. I'd guess it's not encrypted at all.
ETA: Going back to the original thread, the whole question of encryption seems to be dodged and that usually means the answer isn't the one people are looking for: https://news.ycombinator.com/item?id=13440921
Scylla, Discord's replacement for Cassandra, supports both encryption in transit (server-to-server within the cluster; client-to-server) and encyption at rest for stored data.
