Much of it is about development practices. Kubernetes cannot scan your containers for vulnerabilities and misconfigurations for you. Kubernetes cannot ensure lease privilege practices in your containers for you. Kubernetes cannot do regular reviews of logs and configurations and security patching status for you. Kubernetes cannot monitor audit logs for you.
With all this said, it's worth taking a look at the guide. It goes far beyond suggesting a few changes to default settings. Perhaps it could have been better phrased as "Hardening the context and practices around Kubernetes" to avoid this confusion.
More like we need a better Dev mode vs Production mode switch. Dev mode would be fairly insecure but would also refuse to run on the internet. Production mode would ease deploy but also "self harden".
This is why development systems need to be as production-like as possible. Otherwise people ship boring webapps that inexplicable rely on running as root in privileged containers and expect prod to enable this.
:)