travelers.

[Smart $]

What is the NEC reference?

Not a matter of NEC, that I know of. It's one they overlooked, IMO.

 

[Smart $]

From KO to KO, so the result is effectively a single hole that all of the conductors pass through.

From the outlook of cutting the notch, it's best to use adjacent KO's. It'd be a pretty long notch if using KO's on opposite sides of the box

 

[Smart $]

Cutting a notch forces the induced current to take a longer, higher resistance path, thereby reducing the induced current to a level that manages the heat.

Not just that, but in the case of one conductor in and one conductor out, both of the same circuit and a slot between them, the induced current is in opposing directions, thereby effectively cancelling the induced current.