Think of the effort required under that culture to feed 20,000 people - basically they would be so busy surviving there would be nobody available for fighting.

It was that kind of provincial thinking and assumption that got Custer in to trouble in the first place. These people were masters of tactical operations. You don't manage to stampede, slaughter and preserve more than a thousand of buffalo at a time, at places such as Head Smashed In Buffalo Jump, without developing a modicum of skill.