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