[HelloUsername]

But how will they prove which Apple account was just wiped from that iPhone?

[John23832]

iPhones have unique identifiers. So do you, via your set of emails used. The email maps to the iCloud account, which then maps to the iPhone. It would be trivial to map a backup to an iPhone.

[HelloUsername]

Ah yes that makes sense, they can retrieve the iPhone's serial number and warrant Apple to handover the associated iCloud backup.