The critical parts are run in C not Erlang. You think Erlang is fast enough to route packets? btw routing packets and running a backend are two different things.
It doesn't make BEAM pointless, but fault-tolerance and scalability (two of the core features of Elixir/BEAM) are also handled by k8s. If you use Elixir/BEAM for these features and deploy on k8s, it may seem redundant to use them both together. Maybe that's what parent is referring to.
K8s is great for service orchestration and horizontal scaling, and lots of people in the Elixir community use it to deploy, while using Elixir itself to implement fine-grained fault-tolerance logic and vertical scaling.
A dead BEAM process can be restarted in a few microseconds, load up some complex state, and keep going. I don't believe the same can be said of a dead K8s service.