When reading the article earlier I too immediately questioned the claim 9.3x64 is based on the .30-06 case ... a much fatter case, and how could one get .375 H&H performance from an '06 case?

No the 9.3x62 is not the same as a .375 H&H. another case oif gun writer calibre creepism.

As for minimum calibre restrictions, the .375 was largely chosen as a client minimum, probably because so many clients can not shoot properly, arriving with rifles just bought hardly or not used, sometimes not even sighted in. How many times do we still read someone buying a .470 NE double rifle for their first African safari and then sell it off immediately afterwards ...

Untold thousands of cape buffalo have been shot in the old days with the wonderful .318 Westley Richards The original ".338/06" shooting .330 calibre 250 gr RN projectiles. Also poor chums unable to afford expensive English rifles used such cartridges as the 8x60S in common Mauser rechambered rifles.

I would make the minimum, a .330 calibre, brass case approximately the length of a .30-06 case, say minimunm of 60 mm, and a minimum 250 gr weight projectile.

Of course when it comes to calibre creepism, I would argue my 8x68S with a 250 gr projectile will outperform a .338/06.