I'd still disagree that this is true if the rider has even a slight amount throttle control and experience. Under pretty much the same theory, if you think about a thumb throttle with an extreme left turn (when leaning off the seat, the angle of your arm/wrist in relation to the handlebars would, in theory, force you to push your thumb further into the throttle, or at the very least, make it more difficult to back off the throttle.
I dunno...maybe it's just me...I've never had problems using a twist throttle on an atv and I wouldn't have it any other way.