Saw him use this axe a couple times in the past, most recently a couple months back on this tour. Hope the leds are still working as they cut so well through the smoke..
Scarf joint didn't cause the crack, and whether or not the guitar had one is kinda irrelevant in this case. It was the fact that there were holes drilled for the locking nut that caused a weak point in the neck. This is extremely common with older Ibanez guitars because of a few factors (angled headstock, thinner neck than most, possibly overtightened nut bolts).
When I was shopping around for my used Ibanez Prestige, I went out of my way to get one without the holes on the backside of the neck. Just seems like a weak spot to me.