Rocks falling off a moving dump truck pose an interesting physics profile. As they bounce, they loose forward momentum, which is inefficiently converted to a higher bounce height. They can still be bouncing 500 feet behind a struck moving down the freeway.
A defensive claim of tailgating (following too close) may be a deterrent, however it is well established that haulers in commerce are responsible for damage from discharge of goods from their vehicles, which includes the tire treads.
Practically speaking, the issue of collecting for vehicle damage from gravel trucks, cement trucks and equipment haulers, all who seem to be at the top of the list for damaging discharges, is a matter of identifying the truck. One has to get closer for that, and in doing so places with vehicle at greater peril for more damage.
Which brings up another point. If one can see the stones falling off the truck, then there is a obligation to reduce damage, which includes following at a greater distance. On a highway or expressway, this means more than 500 feet and practically 1000 feet or more behind the offending vehicle.
Back to the question, the disclaimer holds little benefit to the hauler. It does not absolve the hauler of any responsibilities. If one is following at 100 feet going 65 MPH, one may get stone damage, and the question may be did one make the situation worse by following too closely.
A sign does not relieve the hauler, who has a duty and obligation to assure that his load and items on his truck do not pose a hazard or do harm when using the highway in commerce. So if one were to make a claim it would be prudent to include the allegation that the hauler was negligent and did not take adequate steps to assure that debris would not be discharged from his truck upon entering the public highway.
Both the hauler, and other vehicles on the road have responsibilities to reduce damage. But the hauler cannot shift the full responsibility to others on the road. And noticing effluent from the hauler hitting the road, others should avoid the debris field and therefore damage to their vehicles.