Seems it was my misreading of the document which is headlined as Class 88 but actually says a lot about the class 68 in its preamble, which is where I saw the bit about having to be rated at 500KW for the class 390.
As for those who don't appear to understand the physics of a traction system as a whole, the ciass 20 has DC traction motors and the system efficiency is about 18% so that what they deliver at the wheel rim to rail is relatively small. An AC traction motor system is nearer 38% efficient so for the same input power it will give just over twice the power at the wheel rim to rail. You also need to take into account the power losses between the diesel engine and the traction motors, there are lots of other parts of the system which all contribute to losses to a greater or lesser degree. At the end of the chain it is the power at rail which matters more than anything else and this is shown more clearly by the tractive effort figures, HP is only a small part of the equation.
Put bluntly, its not the number horses which come out of the engine stable, its the number which get their feet on the track doing the work of moving.