Where did this isCross come from? 😅. It's way too vague like this... What about other cross types, like Canadian cross?

We have three solutions to this problem

1. Canonicalize all pairs so that if they match, we use only one of the values in both places. Works well for Nixpkgs and its implementation.
2. Remove the functions.
3. Don't use ==.

2 is in progress IIRC, but is a slow deprecation. You're doing 3.

1 is a bit more effective though, because it fixes all comparisons of nixpkgs.*Platform.
Could you try add something like this to line 211?

- apply = lib.systems.elaborate;
+ apply = inputBuildPlatform:
+   let elaborated = lib.systems.elaborate inputBuildPlatform;
+   in if lib.systems.equals elaborated cfg.hostPlatform
+     then cfg.hostPlatform  # make identical, so that `==` equality works; see https://github.com/NixOS/nixpkgs/issues/278001
+     else elaborated;

(Perhaps relevant are also #237512, #254763 and related PRs/issues, if you haven't seen them)

That all said, all other modules shouldn't read from anything other than the pkgs module argument, which is the single most reliable source of truth. Unfortunately we can't enforce that; would be simpler.

@Mindavi

Where did this isCross come from? 😅. It's way too vague like this...

From me. I don't think it is vague.

What about other cross types, like Canadian cross?

Nixpkgs is not a compiler, so Canadian cross does not apply to it. Its configuration only has two platforms: build and host; no target.
Individual packages may well support a Canadian cross, but this isn't about individual packages.

@roberth that makes sense, and is also more forgiving to potential usage of == for comparing these platforms, applied your recommendation