Note that they were running Postgres on a 32 CPU box with 256GB of ram.

I'm actually surprised that it handled that many connections. The data implies that they have 4000 new connections/sec...but is it 4000 connections handled/sec?

32 vCPU, meaning an undetermined slice of a CPU that varies depending on what else is running on the box (and the provider has an incentive to run as many VMs on the box as possible).

It’s likely an actual CPU would’ve handled this load just fine.