Winds or mountain waves cannot explain why this plane hit the ground with zero apparent forward velocity.
FWIW, my rule of thumb for a 500fpm up/downdraught is 1000ft above the peak(s) for every 10kt of wind aloft.
Crossing the Alps, I would normally be at FL160-190 which tends to be at least 5000ft above the terrain. I have never seen any significant turbulence. The worst I have had was a boat-like ride which was disconcerting to my lady passenger but, with the autopilot holding altitude, there was no apparent variation in speed.
Normally, entering e.g. a downdraught, and using the AP to hold altitude, you see a airspeed drop and a pitch-up, obviously. I've seen plenty of those, and also the opposite. But I don't cross the Alps in poor weather, and neither was this pilot. Also, the ridge was some way from the nearest other ridge and I can't immediately see how one could end up where he ended up purely as a result of turbulence (a rotor, etc).