Comment by BlackFly

The very difference between somewhat and full homomorphic encryption hinges on this leakage as explained in the paper. The definition of differential privacy as well. They directly admit to leaking a certain amount of information by stating that they apply differential privacy with those given parameters. The issue I am talking about is that such concerns are applied on a single query but correlations across query (the things that actually happen with metadata) aren't considered in the delta-epsilon differential privacy model, by definition.

So if you are curious, just read up in general about differential privacy, or this classic: https://gwern.net/death-note-anonymity Discussed here (possibly in other places too): https://news.ycombinator.com/item?id=9553494