Well, yeah.
Nix is attempting to be better than containerization.
Saying "improvements aren't necessary because we already have 'good-enough' technology" is a meaningful argument when the improvements aren't significant.
In my view, they are significant because Nix can be used to create a fully featured OS instead of just a VM.
Yes buildkit can do this. You can also use buildkit to create a bootable VM, just that nobody is doing it. You can use estargz to fetch just the pieces you need from a dependency rather than the entire depdency as well. Really all of the things you mentioned should be possible with buildkit, just that the focus of most things is Dockerfile which has much more limited functionality (though some of the things mentioned above still apply to Dockerfile).
> they are significant because Nix can be used to create a fully featured OS instead of just a VM
Look up Bootable Containers project by RedHat [0]. Fully featured OS built from a Containerfile, bootable on bare metal.
I agree that Nix design is much better than Docker, and has a bunch of features that OCI ecosystem doesn't (e.g. remote builds[1], partial downloading of the build tree, non-linear build process[2], nix store import/export, overlays, I/O isolation, much better composability), but "creating OS instead of VM" [did you mean container?] is not one of them.
[0] https://github.com/containers/bootc
[1] You can use DOCKER_HOST, and I'm happy that this option is there, but Nix does it better.
[2] Perhaps with BuildKit it's no longer true, I haven't checked what happens if you have multi-staged build with one stage depending on multiple previous ones (which are otherwise unconnected). I think Earthly can parallelize this scenario https://earthly.dev/