Comment by DebtDeflation

I'd say the odds are 50%. The odds of the opposite scenario - where the pilot who said "did you flip the fuel cutoff" wasn't the one who did it are also 50%.

Based on the cutoffs for both engines being flipped 1 second apart, the above exchange being caught on the CVR, and then within 10 seconds the (presumably the other) pilot switching them back to Run, it's pretty clear that this was a deliberate act.