I think you guys are misunderstanding “flex.” I don’t use this kind of device, so I’m not defending it, but there is a sound theoretical basis.
You can hold an iron bar in your hands and stick it out the window of a car going 70 mph and you will get vibrations, or flex, in the system. Sure, the iron bar might not be flexing but there is flex in the system. Hold one foot of that bar out the window and you will have less flex than if you put three feet out the window.
Put your 800mm on the sturdiest tripod and head you can find and expose it to wind and you will have movement. Call it harmonic resonance if you want, but it is flex in the system. Now, rest your hand on the lens hood and you will find the vibration reduced. The more points of contact you have the less vibration.
This type of device adds more points of contact to the head/lens subsystem. It reduces the effective leverage the wind has on the lens. A small thing? Yes, but so is worrying about mirror slap…