As I mentioned when out of the saddle there is an issue, but when in the saddle having the weight as close to the rider as possible has the least effect on bike handling. With the saddlebag tight up against the seat post the rider does not have to fight against front panniers down low when turning or feel the tug of rear panniers. Of course it's critical to have the load well secured to eliminate bag sway as that can be very disruptive. I sometimes see bikepacking saddlebags that are pretty loosely attached and stick out a long way behind the rider and think that it must be hard to ride like that. With the transverse saddlebag tight up against the saddle sway can be eliminated and the mass kept very close the rider's butt. This is indeed all simple physics.