You are right. It's unfair, I'm going to modify the post to mention alpine.
The post is both about tiny images AND how to build docker images with nix. I think it was an interesting tooling to make for our community. Some Nix people are already using it for obvious reasons.
I commented on the original post, but actually I believe the strength of the Nix-based approach is to provide package management capabilities outside of the target image. In other words, it makes the "scratch" image actually usable.
So now, if you take an Alpine-like approach to the problem (musl, no extra stuff) in Nix, you can get much smaller images. And the reason is you don't have to pay for the limitations of the Dockerfile-based approach.
Now, the numbers start getting a little bit meaningless (although that's still an order of magnitude...), but the point is that regardless of how great Alpine is (and it is definitely great), as a base image for a Dockerfile it'll always contain way too much stuff compared to what's really needed for the application itself.
The post is both about tiny images AND how to build docker images with nix. I think it was an interesting tooling to make for our community. Some Nix people are already using it for obvious reasons.