That was true of all the proprietary British big game cartridges. The British were smart to keep their cartridges at 40 kpsia.
Derived from British proof standards, the .470 Nitro Express (NE) has a maximum average pressure (MAP) assignment on SAAMI equipment of only 41,000 psi. American's wanted horsepower, and to get the performance they want, the .458 Win. Mag. has a MAP of 60,000 psi. I have read on other forums the propensity of factory 458 Win Mag cartridges to stick during African hunts. Winchester wanted to shoe horn a big bore, high performance cartridge in a 30-06 action, and the only way to get the velocities they wanted, was to go high pressure. The higher the pressure, the less margin for error. The British custom of increasing the cartridge capacity, to get those velocities at low pressure, is simply the better way to go.
I also believe there can be another cause for extraction difficulties, and that is the shooting community thinks ammunition/gunpowder is immortal, unchanging, pristine throughout eternity. Gunpowder is not immortal, pristine, unchanging, gunpowder pressures rise with age, and the stuff has an unpredictable shelf life. Eley guarantees function of their match rimfire for 10 years, and match winning capability for around five. They don't guarantee function for 20 years, or 40 years, or longer. Combined with the propensity of shooters to hoard expensive ammunition for decades, that may be the reason for the "caking" of power in 458 Win Mag cartridges in this thread:
Why I No Longer Defend The .458 Winchester Magnum
https://www.africahunting.com/threads/why-i-no-longer-defend-the-458-winchester-magnum.17615/
I think what they are seeing is evidence of powder deterioration, such as these individuals experienced:
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old IMR powder, some of it turned "gummy"
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An individual who buys African cartridges, at $70.00 a box of 20, is unlikely to shoot all of it up busting cans. And then because shooters have been taught that gunpowder lasts forever, unfired rounds will sit around for decades. I think it highly probable some decades old, high pressure American ammunition made its way to Africa, and given that the cartridge originally was a 60+ kpsia round, now the deterioration, and heat, rounds could be producing 90 kpsia in the field. Maybe even more!
Regardless of how it is defended, there is no up side to ignorance.