That factoid is from a decade or two ago, when clear air turbulence was a lot rarer. Nowadays, due to global warming, turbulence coming out of nowhere is more common, and on occasion results in unbelted passengers being thrown into the ceiling and severely injured.
I mean, it is good to be empirical about things, but it would fit well into the other evidence we have.
The warmer air means there's more energy kicking about in the atmosphere and, to my knowledge, we have pretty clear evidence that this causes more extrem weather events to occur. For example, hurricanes are more likely.
We'll probably see those on the weather radar to avoid them, but at that point it would be weird to me, if the occurrence of lighter winds wasn't also more likely in places we don't avoid.
I guess, a reduction of turbulence injuries might've taken place independently, because our instruments for predicting them are getting better, but then their frequency would've still increased.
There have been a few events in recent memory that made international news of passengers injured due to turbulence including someone who I believe died.
I am well aware of these events. I am referring to the point about warmer atmospheres causing more clear air turbulence. I don't believe there is conclusive evidence of this yet, though it is a logical explanation.