Even if the motor is stone cold, it should not require fanging the throttle to keep it running. In fact, with my own bike, I let it idle with the choke on before I rev it. Even if that motor is fitted with a GP carb, there is a slide which richens the mixture. I don't like revving a motor which is cold inside - might damage a piston.
I've had a few friends who owned Manx Nortons. I've never heard a Manx which sounds like the one in the video. Lean is usually good, but too lean is bad. If my bike sounded like that, I would expect it to damage itself internally.