This seems a bit of a non-sequitur. Falling from a bike is enough to cause head trauma. The claim here is that a helmet is useful and effective, not just from a moving bike, but from a stationary one. Adding to that, merely mounting a bike is already a greater risk of falling than most folks do.
Is it a certainty? Of course not. But, by that same argument, seatbelts are useless the vast majority of the time. Indeed, on most moving vehicles I'm on (busses/trains), I don't wear them. Not even offered in many cases. I still support and recommend them where they are available. Same for helmets.