Did I miss it, or did they not say why they picked CosmoDB? Postgres has also sharding, so instead of moving to a different DB they could have added a new postgres instance with sharding for the new requests.

Might've had something to do with explaining behavior to their app teams. "Use this new DB product where it's sharded" might be easier than "here's a new postgres endpoint like the old one but now if you join on user ID it's inconsistent".

(not that that's an excuse, but i've seen similar things before)