Even though Leaf dealer guy was obviously an idiot saying the car has 30kv in it, he has a point with the cutting. If the wire is live and cut, the metal tool to cut will short to the other wires also being cut at the same time. Unless you think the vandals were nice enough to cut each wire separately. When that happens 12v can go down all sorts of wires that should have it. It just takes 1 of those wires to be easily damaged and then that entire ECU is trash. Most of them nowadays don't have individual wire protection for over-voltage and over-current protection. And the Leaf, like most modern vehicles is always on. That's how the radio keeps its presets, the windows keep the auto function, the smart key system works, the brakes can prime when you're near by as well as the dome light, the cellular functionality, etc. The car never turns off, only hibernates and more damage can be done.
On the old cars, when you turned it off, it was off except the 1 live wire from the battery to the alternator and another to the ignition switch. That was it. Cut all the wires you want (except those) and nothing bad would happen. Vehicles have more bells and whistles today.
This is a good reminder why I choose to keep full coverage on every one of my vehicles including the clunkers.
2013 Nissan Leaf SV with every option
2013 Prius C Package 4
2012 Lexus RX450h
2006 Prius Gen II, pIV
2005 Honda Civic Hybrid
2001 Honda Civic EX
1997 Honda Prelude SH
1981 DeLorean DMC12