Indeed the acceleration must occur, but starting at 50k feet substantially reduces the amount of air the rocket has to push out of its way. This not only makes

Indeed the acceleration must occur, but starting at 50k feet substantially reduces the amount of air the rocket has to push out of its way. This not only makes acceleration easier, it also dramatically reduces the dynamic pressure on the vehicle at max Q, allowing for a reduction in the mass of the vehicle (making acceleration easier still).