That is a good point! I forget that Apple tends to deprecate and remove things over time, whereas the 32bit Windows and Linux builds will still work.
The Mac OS X build is for 32bit Intel.. I've now specifically marked it as such to avoid confusion, since you are correct in that it no longer works on modern macOS systems.
Grim is almost ten years old now! I can't promise I'll be able to update it to work on newer systems any time soon, but I will have a look into it when I find some time.