I was not there so it's hard to comment. I know I never have problems with the Ki-84 and I know the numbers say that the Spit 8 is a better plane.
I think what happened was there was no one who really knew the plane and ki-84 is a better turnfighter (especially on the deck), therefore the Ki-84 pilots had a large advantage.
Ki-84 is much snappier on the roll than the Spitfire VIII , is tougher than the Spitfire VIII and is a bit faster than the Spitfire VIII down low. I also wouldn't undersell the Ki-84's guns. They are very good, perhaps even excellent when it comes to Spitfire killing.
In that AvA setup, the Ki-84 did a very credible job of dominating the Spitfire VIII. It was very lopsided, not just a little bit.