Is it just me or is the reasoning behind the switch from postgres to mysql very vague? They describe a sharded mysql database... Sharding postgres isn't necessa

Is it just me or is the reasoning behind the switch from postgres to mysql very vague? They describe a sharded mysql database... Sharding postgres isn't necessarily any more difficult, instagram apparently uses it in a sharded manner with many shards. You'd think storing json in the pretty sweet jsonb column type in postgres would be a nice bonus for querying or indexing on. I guess someone at uber must really like mysql, a good enough reason as any other I suppose. I'd love to hear about what other reasons as to why mysql turned out to be the choice here, as I've usually gone the other way (mysql to pgsql) for many of the great features and performance pgsql has.