Not sure how I acquired the "common" knowledge that a plane has never crashed before due to turbulence alone, but this looks like an example of such a case!
I think maybe you've misunderstood what the common knowledge said. Turbulence hasn't ripped an airplane apart at altitude, so yea turbulence alone hasn't caused a crash. But anything near terrain is plenty risky (mountain waves, low-level wind shear). If the turbulence makes you drop a couple of hundred feet that's no big deal at 35,000 feet and no matter how bumpy it gets, the wings aren't going to break off. But if you're near the ground (or a mountain), well....
I guess it depends by what you mean by "at altitude"? Most of these accidents did happen below 10.000 feet yes. And the mountain wave phenomenon in New Zealand in particular reaches pretty high. Some glider pilots use it to get up really high, so high some of them got frostbitten.
The good news is that very strong downdrafts, in absolute terms, simply can't happen close to the ground, because the air in question needs somewhere to go.
