Ah this lateral flex is something i noticed with my Bitranz 65mm carbon spokes wheels too. I think its mainly due to using only 20 spokes or less. But yes, thinner side walls in the rim profile because you use less material with better performance lead to a smaller area moment of inertia and the geometry stiffness, which is represented by the multiplication of the Young modulus (200GPa for normal carbon fibers, but 350 to 380 on high performance carbon fibers) with the area moment of inertia.
This means if you use less layers of carbon, the layup of unidirectional fibers is also resulting in less fibers oriented for lateral stiffness. I attached a drawing for the directions of layup, that everone knows which directions i talk about. Against lateral deflection the layup needs to have a good amount of fibers oriented in some degree like + and - 30 degrees. When you only use 0 and 90 degrees on the side walls of the profile you get a great load path for pottholes and impacts, because the 90 degree fibers take a lot of force and the 0 degree would act more like a bond for holding it together around the wheel.
But as i said, using less and less spokes is also a big factor because the spokes absorb the force in the end by reduced tension on the side bended towards and a higher tension in the spokes against the bending. and if the distance between the spokes becomes greater the more leverage is there what you can deform.
