Not so.
The 90-650 was the original “Full Nitro” load at the turn the century. It is noted clearly in the later 1910 H&H catalog.
.577 90 Cordite - 650 Grain bullet was offered in 3” and 2 3/4” rifles and gave a claimed 1950’/sec out if 28” barrels.
The .577EX stamp did not denote solely Black powder proof; for Express Rifles “if” accompanied by the Cordite charge and bullet weight it was “nitro proof” until the NITRO “NP” stamp was introduced in 1904.

The .577 NfB loadings were usually 75 grains of Cordite.

Graeme Wright, Wal Winfer and Hoyem all note the 90-650 loads.

As to whether the 100-750 loading is “safe” in a 90-650 proofed rifle is up for debate. Supposedly the proof was done at 100-750 pressures whether or not it was stamped at 90-650. This tidbit is mentioned in the notes on page 203 of the 3rd edition of Graeme Wright’s book “Shooting the British Double Rifle”

- Mike