Which is just managed PG Citus with the word Cosmos in the name it seems.
It’s worth noting how cloud vendors have fallen back on tried and true databases such as Postgres (mind you, often replacing the guts of those databases with new implementations.. but still).
Have they? Amazon created Aurora from scratch, with compatibility layers for different database engines (MySQL, Postgres), and GCP did the same with Spanner, which i wouldn't call "fallen back".
That said, you didn't mention things like RDS or Google Cloud SQL, which are cloud-based versions of standard open source dbs.