There is metadata; whether you consider it interesting is up to you. The tarsnap client has to say "I'm machine X, and I want to store a block of data with tag Y"; and when you extract an archive, "I'm machine X, and I want to retrieve the block of data with tag Y". This could allow someone to figure out (a) that it's the same machine, and (b) that you're extracting an archive which contains data which was stored at a particular point in time.

Paranoia means encrypting everything which might be sensitive, even if you can't see any way for it to be abused.