BBN radar, I find your scenario hard to believe. First off the D8 fleet has has over 20 million hours, this would have happened a lot more times, and we would know about it. This engine/ propeller system is used on other aircraft and as far as I know, what happened here has not manifested itself as being a problem. How many times has a crew oversped an airplane with this engine/ propeller combination, what would they do first if they're overspeeding and they have the power levers forward? I'm sure pulling back power in response to an overspeed is not unusual. If this is some kind of anomally that shows up once every 10 years, why did both props overspeed at the same time?
One thing that does come to mind here is a malfunction of the gate itself. The gate is spring loaded down, if for some reason it wasn't down and the crew pulls the PL's back to flight idle and the gate isn't there... Anyway, the beta lockout system on US registered D8's is there to prevent this, it should mandatory worldwide.