That's how I understood it too.

At the time when Russel comments on the use of the cartridge 577 Express, the cartridge 577 Nitro Express already existed, but he still only speaks about BP cartridges. It was known that in India hunters held on to BP rifles for a very long time; apparently such rifles, especially caliber 500 Express, were made to order until the twenties of the last century.

All of this to emphasize that these BP cartridges had a certain efficiency when hunting big and dangerous game. It certainly no longer correspond our current standards when it comes to cartridges and especially bullets for hunting big game.