Okay, so I downloaded it again, but this time from a different website called Visit Mama, and it works. So I suppose its Itch thats screwing up the download because the file already came as an executable.
But still, thank you.
Viewing post in MAC Game immediately closes upon opening
It's probably me uploading on itch.io incorrectly, as I use Butler. My guess is that something happens to the archive during the uploading process which corrupts the executable within, similarly to what happens if I try to zip a Mac game on Windows. I'll be using tar format for packing in future, and hopefully that'll fix the issue.