Let us disregard the actual wording of the rules since it is subject to interpretation. Lets assume the wings are flexing.
1) Do we have any evidence beyond photos proving that RB flexes more than others?
2) Can't it be argued the RB employs rake to get their front wing lower?
3) Most importantly, if it is flexing more than the other, then how is it done? Allow yourself to model any solution that would generally fit within all the other F1 construction rules. At this point completely disregard form factor or overall shape. A model than bends non-linear to a linear applied load.
Could it be if it is not possible then it is not being done?
Brian