Shared SQL databases tend to have very poor performance when you join across shards. In this case the downside of distributed joins is avoided by having very many small purpose built databases.
All IO is synchronous? Writes don't actually write when they say they do? Running 201 queries is as "fine" as running a single one with join?
It seems as they got every bad ideia in the book and somehow made it into something that works (that is, if it actually does).