The article discusses what you get by using K8s alongside Fly.io. If you want to bin-pack containers onto Fly Machines, you can of course just boot up your own K8s cluster here; that has always been an option.
This is a critical feature available in every Kubernetes offering, and as such people rely on it. Trying to say that maybe you can do without is missing the forest for the trees.
Like saying that your C compiler doesn't need arrays because it has pointers: sure, maybe, but now good luck compiling any existing code. Maybe don't call it a C compiler if no C program will work on it unmodified.