Just did a quick switch-a-roo to put this in nativeBuildInputs (see this answer for why). Thanks!

Just did a quick switch-a-roo to put this in nativeBuildInputs (see this answer for why). Thanks!

Could you elaborate on that? Because I always use buildInputs for all my devshells and have never had problems with that and the post seems to suggest that it just doesn't make a difference

Sorry, totally missed this when it came in!

So, my answer was about to be that outside of cross-compiling, there are other distinctions when it comes to *Inputs such as in regard to shell hooks. However, that means I missed the literal first line which states clearly that for mkShell it doesn't matter. Looking in the source for mkShell, that tracks, everything is outputted together: nixpkgs/pkgs/build-support/mkshell/default.nix

This also reminded me of the existence of inputsFrom and packages. packages is the mkShell-specific attribute that I would wager was added in order to side-step the confusion of *Inputs in a shell, I would recommend using that for most cases.
inputsFrom is very special in that it lets you use the dependencies from a package, the reason we don't use it in this shell is because it doesn't include the transitive dependencies needed for Qt to work (and which we have to wrap the Python executable for, which that wouldn't do either). That, and what it would include is all the Python dependencies from nixpkgs, which in this case is undesirable. Otherwise, I would recommend it.

TL;DR: I was wrong, and I am switching the shell to use a single packages list.