120V (or even 240V) in common mode should do nothing. 120V in differential mode will cause massive overcurrents in the circuitry adjacent to the port and possiblly in the switch as a whole but it's unlikely to be able to go in one port and come out another while still having enough strength to do damage to devices beyond the switch.
However TFA was clearly using a LOT more than mains voltage. A sufficiently large common mode voltage (you can't really apply very high voltages in differential mode because your connector will just flash over) will either punch through or flash over the isolating transformers and find it's way to ground by any means nessacery frying stuff along the way.
Cheap switches are probablly going to be worse than expensive ones because they are less likely to be grounded (meaning the only earth path is to go beyond the switch into devices behind it) and more likely to have isolation transformers that don't actually meet the requirements.