Regular 9 1/2s are pretty hot as it. Wouldn't surprise me if that was the recommendation vs a magnum.
Yes. the 9 1/2 is a very hot primer indeed. However, H414/W760 seems to be easy to ignite. I did a side by side 308 Win range test around 10 years for a few powders using batches of standard Lapua large primer brass v the then new SP / small flash-hole Lapua 'Palma' variant. H414 was the sole ball grade tried and to my great surprise performed better in the SRP brass than with a standard large rifle primer. Better as in slightly higher MVs, smaller ES values and smaller 100 yard groups. This was on a chilly day with ambient temperatures in the mid 30s / 3-4 deg C.