So I just learned that apparently flake.nix files aren't actually proper Nix files, despite having a .nix extension, and that they introduce additional syntactic restrictions.
Not type checks on the evaluation result - *syntactic* restrictions. A flake.nix that consists solely of an import of a valid flake under another filename, would be invalid, even though it would be identical value-wise to the valid flake itself.
Great. Because what Nix sorely needed was definitely more hidden footguns!
You know, this whole thing wouldn't have been an issue at all if they'd simply given it a different extension, to make it visually clear that it only permits a restricted subset of Nix.
But apparently nothing was learned from the existing mountain of footguns.