"Zoom" is different from sustained climb regardless of how much speed you start with.
A high drag/lift aircraft may have an enormous sustained rate of climb, but the high drag and lift may hamper its ability to trade speed and altitude at will.
Simple example - take a brick and a wad of paper. The brick is the jug and the paper is the spit. Toss both upwards at 20 mph. The brick will "zoom" a certain distance, the paper will "zoom" a somewhat shorter distance.
This example is extreme in that neither the brick nor the paper have a sustained climb ability, but it doesn't take much of an imagination or even complex math to extrapolate that a plane that flies like a brick (P-47) may zoom better than a plane that has the drag/weight characteristics of a wad of paper (spitfire). The P-47 may have a better zoom, better dive acceleration, and even higher top speed depending on drag and horsepower, but the spit being optimized for turning performance with lower weight and higher lift will likely have a better sustained climb.
So BOTH historical accounts are likely correct and the practical application of either performance characteristic is a matter of timing. The P-47 driver may only have a moment in which his superior zoom provides an advantage. Wait too short of a time and you haven't fully exploited the better zoom, but wait too long and the spit is beyond "zoom" and is simply using it's climb rate advantage to eat away at the Jug's already-used zoom advantage.
Climb rate at high speed may however be a different story. Take a spit flying at max sustained level speed and a jug climbing at that same speed, and you find the corner of the envelope that the Jug can actually outclimb the spit. The P-51 in AH is outstanding in this regard (look at climb rates at 150, 200, 250, 300, and 350 knots and you'll see what I mean) and the Jug also has some advantage over slower but typically "better climbing" aircraft.
Of course, using that advantage makes you look like a running coward, but war is hell...
