Oi!, Thats not what I inferred at all, and is not what happens in practice....
The cartridge has two output pins. One of them is connected via the bit of UN-screened wire to a screened cable "inner" that goes to the "live" side of the amp input. The other goes via the un-screened lead to the screen of the cable going to the amp.
It is NOT connected anywhere on the deck at all. Its totally floating..
The arm deck metalwork can either be "earthed" via the mains earth and also it sometimes is earthed to the amp via a single earth cable.
If that is done we still won't have a hum loop as the cabling going to the cart back to the amp is floating and is not part of an earthed circuit there at all..