The difference is in the mass of the taller deck for the RB block. It's the equivalent of having a pair of torque plates bolted on top of the B block. I dont think anyone ever said the difference was in the cylinder wall thickness....
I see you didn't read the entire article. He listed the weights of five blocks that he had on hand. The difference between the lightest and the heaviest was about 16 lbs. His 383 and 413 blocks weighed the same. I suspect the B is not much lighter than the RB due to the cylinders on the B extending farther into the crankcase. I think the main performance benefit of a B vs a RB with the same stroke may be the lighter reciprocating assembly. And there is a consensus that the 400 is the sturdiest block.
