Well a quick google turned up this:
http://rapidttp.com/milhist/vol032rm.html, a nice comparision of the two eeriely similiar battles. Note that the author doesn't share your opinion that the native units were better armed than their more advanced counterparts. Still looks like many impi/horse archers units weakening and then destroying riflemen/cavalry units to me. If Shaka had only whipped out a couple more impi he'd a been able to take out Victoria's defenders at Rorkes Drift too, sounds like they were down to just a couple of points.
This statement here from your link is extremely inaccurate:
"There was a larger variety in weaponry available to the North American Indians, but none of it was really superior to that of the Zulu."
The Sioux *ORIGINALLY* had primitive weapons, but over time they acquired rifles which allowed them to hold their own far more often against the U.S. Cavalry, than before:
http://www.eyewitnesstohistory.com/custer.htm
"As the Indians closed in, Custer ordered his men to shoot their horses and stack the carcasses to form a wall, but they provided little protection against bullets."
BULLETS. Not only bullets, but bullets which penetrated entire horse carcasses and KILLED the man hiding behind it. These were no ordinary bullets but highly powerful rifles firing them.
Another key clue from a Lakota account of the battle:
http://www.pbs.org/weta/thewest/resources/archives/six/bighorn.htm
"The coming of the walking soldiers was the saving of the soldiers on the hill. Sioux can not fight the walking soldiers [infantry], being afraid of them, so the Sioux hurriedly left."
The "walking soldiers", non-cavalry, had as a part of their formations, small artillery guns, and much heavier, more powerful, longer-range rifles. This was why "Sious can not fight" them: out-GUNNED.
By contrast, the U.S. Cavalry was armed with the lighter Winchester rifles, less powerful, less accurate, and more geared for rapid aim-fire at closer range during a high-mobility charge.
The way the Sioux acquired rifles, over time, was largely battlefield capture. From the same link:
"These different soldiers discharged their guns but little. I took a gun and two belts off two dead soldiers."
Interestingly, another element the Sioux used to advantage at Little Bighorn, that I learned just now, is the element of stealth:
"The Sioux men took the clothing off the dead and dressed themselves in it. Among the soldiers were white men who were not soldiers. The Sioux dressed in the soldiers' and white men's clothing fought the soldiers on the hill."
So at some point in the battle Custer's men were surprise-attacked at close range by Sioux warriors dressed as U.S. Cavalry (and also armed by U.S. rifles taken off of fallen soldiers, both at the present battle and at previous battles). Interesting trick for "primitive plains Indians" hehe.