Are they using Nix wrong? Or is this seriously a limitation of the current state of the tooling?

If it's a limitation of the tooling, that almost sounds like satire. I.e., someone builds a powerful functional DSL, uses it to painstakingly construct a monstrous package graph, then gets tired and wonders, "Wouldn't it be great if there were a language and data structure that were both custom suited to automating the grep'ing of argument types for my DSL's package graph..."

Of course, sometimes those kinds of changes just never get merged, for a variety of well-understood open-sourcey reasons, eg: https://github.com/pocoproject/poco/pull/3105

But in short, the maintainers are mostly uncompensated, and therefore have no sense of urgency or obligation to a faceless stranger making a drive-by contribution. Every accepted contribution is something that has to be understood, supported, and even justified by the project's maintainers going forward, so each one is an opportunity to take on more work and risk, and obviously by definition almost all them are things the maintainers didn't themselves "need" or they would have done it themselves. So each new branch or bugfix is relevant to a codepath that no one involved in the planning, testing, or release-preparation for that project has a first-class stake in. Each new capability added is an opportunity for there to be shrieks following a release because it was inadvertently broken in the course of some other bit of development.