Obviously you don't race in real life. With the gearing he had 3rd gear meant he didn't have to change gear for a lot of the course, and the car was still traction limited even at lower revs. To change gear then would have been pointless and slow.
Watch the video - in 3rd gear out of the corners he gets power oversteer. Not caused solely by power, but power whilst cornering. Hence traction limited, rather than power limited, when full throttle just accelerates the car.
Not for you, bug in general: people, please remove that "nz" (or any other language) thing from the url before posting the link. It changes the language of youtube and it's annoying, especially if the language is something else than English.
My Dad knows that guy Graham White Jnr. I've seen that car in person! Last I knew it was/is powered by a former Arrows F1 car engine but he had too many technical problems with it so he was swapping it for a engine from a DTM car.
I've posted this before (elsewhere on the forum), but my link is dead now, since it was a .wmv from brayspeed.co.uk. You have to register on the site now, but I recommend going through the dull rigmarole of signing up since they have loads of good quality hill climb footage (have a look at 2t4t too)