I think it's you who is missing the point: the learning environment won't build as neither docker nor vagrant will give you reproducible environments, while nix will. Sure, you can make your nix environments dynamic too, but having determinism is its core feature. I don't think it is with docker nor vagrant, as the environment you get will be isolated, but not deterministic. You can, of course bolt some custom thing on top of those (or use nix), but that is going to be a pain.
EDIT:
Both vagrant and docker are useful tools, it's just that for creating any dev environments nix will be better due to determinism.
The point is, and always was: someone wants a box to experiment in. Nix is not that box. It was never the box. It will never be the box. That is the point I am making and reflects the point the author was making.
And just in case there is a language barrier: box does not refer to a specific technical implementation of a box, it's just a term to denote a border between the user's system (which they don't want to break) and "something else".
Edit: and just in case the word 'someone' trips over a language barrier, I'm not referring to 'anyone' but to the persona (the 'someone') who might want to try something out because they saw something and thought it was cool. Not someone with package manager experience, not a software engineer, not a sysadmin.
We're also not 'building a learning environment'. Learning environment is a proxy for disposable environment, which as a relatively simple concept is already a step too far for the average "how do I become a cool hacker like on TV" case (which is where the unreasonable effectiveness from the title comes in) where someone might take their first steps and wanting to try something out without breaking their current environment.
Not in this case. In this case we want a general computer user to perform an action to have a 'computer in a box' that they can break with no consequences with a very low barrier to entry. And we want that action to be easy to share.
Nix also gives you (not me, just in case you are going to assume that) a headache, which is why it's not getting the adoption that everything else does get. Nix is the betamax of state managers.
Docker gives you the headache of not actually giving you reproducibility. If you haven't been bitten by it then I don't think you have tried.
For popularity, I wouldn't actually mind if nix weren't all that popular, but it may just have the largest number of packages available for install for any package manager.
I haven't heard of anything getting a Docker-related headache because it delegates reproducibility to whatever tools you pick. We haven't been bitten by it, and you might think I have not tried, but you'd be wrong and making some odd assumptions.
Perhaps I can assume that you have only used nix personally or at a small company?
EDIT:
Both vagrant and docker are useful tools, it's just that for creating any dev environments nix will be better due to determinism.