The overhead is actually the conversation we're having right now about whether postgres or Redis is better. It's not that postgres is hard to use or less perfomant, but that there's memory overhead in "Here's how you use Postgres for session management, here's how you use it for application building". Use Redis for this, Postgres for that is easier to grok.
That's not the scenario they're describing, postgres has most likely already been designed and worked on to scale to their workload, using postgres means you don't have to replicate that for another system.
For Redis though, the overhead is far more trivial than something like Kubernetes or Kafka, or even Elasticsearch or MongoDB