Reference "no wx, clear skies."

Just thinking out loud here. 8-9 yrs ago, a Polar 747 flight from Singapore to HKG, cruising at FL390 in "no wx, clear skies" conditions suddenly hit extremely bad CAT. Jet flipped 120 degrees inverted and went into a massive high speed dive, almost Mach 1. My buddy got lucky, jet stayed intact, recovered plane at 15,000ft and limped to HKG. Don't know, just opining.