There are many private, proprietary systems that exceed those scales. They are usually bespoke to the applications that they run. I work with two such systems and know of others in finance, energy and scientific computing. Not to mention Borg at Google.

In the commercial proprietary world, clustered mainframes and supercomputers have addressed this niche for decades.

In the open source world, HashiCorp Nomad is the most analogous alternative to Kubernetes on commodity hardware, while SLURM is very successful for supercomputing.

I've also scale tested k8s to 15k nodes (in a limited configuration for a single application). At that point we ran out of underlying hardware budgeted for the test.