Not an unusual occurrence in wartime. I suspect the Army also disarmed captured German soldiers in WWII.
The Sioux were at war with the US, as can be seen by the acquittal for this reason of several Sioux warriors charged with murder for killings committed during this period.
There’s a lot to learn from what happened at Wounded Knee. It was not a shining example of US Army good behavior.
But it has very little applicability to today’s 2A issues.
Well one thing to learn would be to not make a circular firing squad.