Having flown a Discus (glider) in glorious wave over Cambridgeshire, I'll agree that wave persists a long way downwind of mountains, given the right conditions..
Two thoughts...
Having recognised that things looked unusual, did the PF give any thought to flying pitch and power, accepting the speed, and thus avoiding the low speed event?
Second, recollections are notoriously inaccurate. You could ask the company safety people to let you see the FDR trace, assuming it was captured by FOQA or similar... It may tell a much less interesting story...