True but the 9.3 was developed in the early 1900s. The .358 NM was developed in the mid-1900s. It is not surprising that it might be a more capable bullet. Just like it is not surprising that a .338 Lapua developed in the late 1900s (or the relatively new .338 NM) can outperformed the .358 NM. Bullet development has improved with time.
The fact that the 9.3 is still legal for the big five is a grandfathering provision more than a measure of its capabilities. Correct me if I'm wrong but new calibers less than .40 cal are being not approved for the big five despite their capabilities which may well be in excess of the 9.3.
Oops. Miss an edit. Should be "are not being approved."