The claim is about 30% on 3,000 hp.
"To do that, the math says they need close to 4,000 horsepower. Yet, stationary dynamometer tests of these engines routinely only register 3,000 horsepower or less."
Think about a ram duct, if such a thing exists. You are a globby thing of air in free flow. You are heading towards something like a snorkel. Do you dodge the snorkel or do you dive down the snorkel?
Because you are a globby thing of air, you dive for the point of lowest pressure with all of your mates ahead of you. One mate goes to the left of the snorkel, one to the right and one straight down. You and your mates behave like that because you (globby things) try to do the easiest thing. You do the easy thing -- and there is no ram effect because free air will always move away from positive pressure.
"With a forward-facing air scoop at lower vehicle speeds, the engine is pulling air into the scoop."
Aah, you have a negative pressure area.