An app doesn't need to be multi-threaded to horizontally scale the physical hardware. A load balancer can dispatch requests to multiple single-threaded processes running on individual servers.
That would result in multiple distinct instances of HN that don't share data. You're thinking of an architecture with a separate central data store that all the instances access and update, but that is not how HN works to my knowledge.
That's just how powerful modern hardware is. You don't need a k8s cluster to run a simple CRUD app under realistic loads. I'm sure the hackernews codebase has acquired some performance optimizations over the years, but I doubt that it's doing anything especially clever performance-wise.