"Default versions" breaking things that depend on them? What is that? It is like using docker's ":latest" tag and being surprised each time that a new server falls on its face because the "default" image is actually a different version from the previous "default" image.
I don't understand any of the explanations in this blog post. Seems like people who have zero clue about what a "version" of a software is.
"no way of splitting up the Nix dependencies into separate layers" - Why? Of course you can split /nix/store into as many layers as you need. Do they even know how to use containers and how to use Nix in the first place?
With the clear incompetence of these people, no wonder that their proposed solution smells like a decomposed fish.
Classic NIH syndrome. There is going to be no surprise to see them meet the exact same problems they didn't solve with Nix to infest their new "solution".