This is my guess of why they didn't use Citus. They weren't interested in the options of (1) going multi-cloud [DB in Azure Cosmos / Backend(s) in AWS] (2) going all-in on Azure [DB in Azure Cosmos / Backend(s) in Azure] (3) self-managing Postgres+Citus in EC2.

It'd be interesting to compare the expected capex of developing this in-house solution + the opex of maintaining it vs the same categories of expected costs for option (3) – because I imagine that's probably the most palatable option.

They also may have pre-paid for dedicated RDS instances for the next X years (before this horizontal scaling initiative began, to boot), as AWS allows companies to do this at a pretty steep discount rate, which would probably tilt them away from (3).

Especially because Option 3 lets you go waaaay farther on vertical scaling, since you can get native NVMe drives (they mentioned hitting IOPS limits for RDS), more exotic instance classes with far more RAM, and do stuff like ZFS for native compression and snapshots.