This isn't true in two cases: diving in current/with a scooter, or in cold water, like we have here in the Great Lakes.
If for some reason a diver is stationary in a current (hanging onto a mooring line, or jamming an anchoring screwdriver into a clay/sand river bottom like in the St Clair River where there is a strong current, while digging out an antique bottle, for example) a free flow can happen. Scootering can cause a free flow too.
But the biggie here is the cold. The water today was 41F. And over the next four or five months it will be even colder. Free flows happen often enough in these conditions, hence I dive with redundancy (sidemount, backmount doubles or single tank backmount with slung tank charged and off).
In water just above freezing, or worse, if the air itself is below freezing, sometimes even breathing through the reg on the surface, without having the reg in the water, will suffice to have a crystal or two of ice form in the second stage. Then pow! Free flow. The cure is switching to the other regulator and turning off the offending tank valve. Sometimes waiting a few minutes will let the water melt the ice, and you can try turning the tank valve on again and things are fine. But often for single tank divers the dive is over, the reg is a ball of ice. That's when slung tanks are vital IMO.
Even hitting the inflator mechanism can cause a free flow if there are water droplets inside it in winter diving.
Just expanding people's horizons. Not all diving is warm water ;-)