[dddd]

I wonder if the guy who told this story might have heard it from a reliable source, but didn't realize that it is mtb thumb shifters that have caused more than a few chronic and severe thumb-joint injuries(?).

I heard long ago that the switch to below-the-bar mtb shifter designs was in response to racers suffering thumb injuries when shifting while traversing bumpy terrain that their bouncing glasses and eyeballs prevented them seeing clearly. Bikes had no suspension in those days, so with the lever oriented in such a range of angular positions during use, could literally cause thumb joint failure under unlucky conditions of lever position, thumb motion and trail impacts.
The newer designs of shift levers returned the lever to the same angular position after each shift, with the lever held more perpendicular to the thumb's direction of force, preventing impact forces from being transmitted as directly.