According to mr. Koopman's presentation, turning off the ignition turns of the steering assist and would be a bad idea. The recommended workaround would be to shift to neutral.

Apparently it was one of those "push button" ignition systems, which you have to hold down for three seconds to kill while in motion. Probably the driver didn't know that, or didn't hold the button long enough in a highly stressful situation.

The "old-fashioned" key ignition would have provided a much clearer interface to killing the engine. As would shifting into neutral, which is the obvious other option; dunno if there was a reason that wouldn't work.