An extreme muscle pump is counterproductive. Why? Because this means the cell membrane has shut down - no wastes can leave and no nutrition nor oxygen can enter. The cell is filled with lactic acid and the calcium cycle is inhibited = muscles can't function optimally. This is retarding your recovery for the next set. Attempting to lift heavy loads with an extreme pump, without lengthening your rest periods, will make your potential workload will suffer. You will not be able to push as much weight, getting less type IIb stimulation, therefore lessening your chance at building larger muscles.
A muscle naturally fills with blood and various other biochemical products/byproducts with the natural physiology of exercise. But, purposefully training for "the pump" is not intelligent training.
I believe CT was referring to achieving a pump from posing to densify the physique, not going for a huge pump during a workout.
Finally, I would love it is folks would try not to quote "Arnold" so often. #1, he used drugs. #2, his genetics were awesome. #3, he knows little about physiology if he is generally suggesting that an extreme pump is productive. #4, he was a bodybuilder, not an expert trainer nor professor; he is not an expert in physiology. Most of the authors on this site that I learn from would probably never get close to the stage of the Olympia if they tried. But there is not one bodybuilder on that stage in the last 25 years that I would take training advice from.