Mythology
Most of the buffalo were dead before the 45-70 was developed in 1873. There were laws passed in 1874 banning the hunting of buffalo to preserve the handful left. There were very few buffalo killed with 45-70, they were hunted with much more powerful cartridges.
In the 1870s and 1880's the 45-70 developed a reputation for being pretty weak and was not considered adequate for game larger than deer. Those black powder era loads were basically equivalent to 45 caliber muzzle loaders. Which is the legal minimum for deer in most places. Not legal for game larger than deer in most places.
Modern loads, in modern rifles are a big step up, but the best loads suitable for a lever action are considerable below 458 WM. You can shoot a bullet about 100 gr lighter to about 100 fps slower than 458. If you step up to a Ruger single shot or bolt action the best 45-70 loads will shoot a bullet about 100 gr lighter than 458 to about the same speed.
That is good advice. When 450 Marlin was introduced virtually all factory 45-70 loads were very low powered loads suitable for the old rifles. The only option to get modern 45-70 loads were to handload. Ammo manufacturers were reluctant to make modern loads for liability reasons. The 450 Marlin filled that need. You got the same power level as the best 45-70 handloads, but in a cartridge that would not fit in an older rifle. There are now several options for factory modern power level 45-70.
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