Contradictory messages about restoring to a spare device My iPhone 4S died and I need its contents (e.g. authenticator app to log in to customer systems). I have a backup from 2019/12/31 on my MacBook, so I want to restore it to an identical spare device we have around in the office. That should work, right?
However... iTunes (12.9.5.5 on macOS 10.14.6) claims

iTunes could not restore the iPhone &quot;[name]&quot; because the backup was not compatible with the iPhone that is being restored.
You must update the iPhone to the latest version of iOs before it can be restored from this backup.

But the iPhone says it's running iOS 7.1.2 and there isn't anything newer.
Which side is lying, and can I force the restore in some other way?
I don't think any of the sides are "lying" (i.e. being deliberately deceitful). I think the person wrong here might simply be you.

You're saying that your iPhone 4S died, and you're trying to restore to an identicate spare device. I have a suspicion that your identical spare device is not in fact identical. I think it's an iPhone 4.

The iPhone 4 cannot be updated to iOS versions newer than 7.1.2. In contrast the iPhone 4S can be updated to iOS 9.3.6.

In your case you will need to find newer hardware to restore your backup to.