It's true, but in all the years of my childhood when I did this (and I rode and turned no-handed often), I never once had an accident when my hands were off the handlebars. Your warning is still useful, I think, but I also think good judgment goes a long way here.
And I even drive at night, without light on forrest roads without hands ;)
So maybe I push my luck too much, but I am also confident in my skills and reflexes ...