Potential and actual usage aren't related. They might be having a lot of records and read/writes but maybe the actual pub/sub isn't that intensive. They seem to be using the same DB for everything

Be careful not to confuse average load with peak instantaneous load. Bursty workloads are the bane of capacity planners everywhere.

Does postgres scale that well? I would be interested in case studies as I've not seen much achieving beyond 10k records per second.

Whose to say it can't?