That is not what they mean at all. It is what several people say, but that is merely ignorance. What they mean to say is that the realistic sound of the P-51 from the cockpit is an unexpected result. Most people think that because the Rolls Royce is so ferocious, yet a true thoroughbred, that it should sound like a race car engine at high revolutions. When they discover that not only are the revolutions low, but that the sound is not so ferocious (though still very, very loud), then they react is though the sound cannot be correct.
Rather than incorrect, the sound has shown that your friend with the P-51 experience is either very, very mistaken, or merely part of your imagination.
Have a nice day.