Apple explicitly suggest an exit(173) if the certificate check fails for Mac App Store apps.
Although I've definitely had non pirate users report this error to me because they've drag and dropped the app onto another machine instead of reinstalling from the App Store -- so you're right, it's not a guarantee that they're a pirate in an absolute sense.
Apple suggests it for Mac apps because you can copy a purchased app to another computer manually, and the exit(173) signals the system to go do an authorization check and re-authorize that copy to run on its new computer if it passes. iOS has no equivalent mechanism.
Although I've definitely had non pirate users report this error to me because they've drag and dropped the app onto another machine instead of reinstalling from the App Store -- so you're right, it's not a guarantee that they're a pirate in an absolute sense.