I *believe* the hard steel - soft stone corelation refers not to the cutting agent itself, but to the binder and it refers much more to natural stones than synthetics.

If you have a very hard steel it will be reluctant to be abraded.
Therefore you have to either increase the pressure or the speed of the abrasive action.
Or you will have to increase the amount of cutting particles.

If a stone is bond soft, it will release many particles during the sharpening process, making the cutting process easyer.
That´s why softer stones propably will cut faster with harder steels,
than harder stones do

There are very few minerals that actually are softer or not significantly harder than 65HRC steel.
Not all of them are able to cut all the carbides in steel,
but this would be useless, anyway, as they will break out during use