I'd agree. He didn't, for two reasons I can fathom: 1) He didn't at the Washita and was successful (although Maj. Joel Elliott, had he lived, might have disputed that); 2) He was concerned many in the Indian village(s) would escape if they weren't surrounded. The first is debatable, the second is a proper argument IMO.
It's one of the great battlefields to visit. It properly takes days to do it right. I happen to have an unfired Indian bullet given to me by a good friend (also a LBH history nut), who was given it by then Park Ranger Don Ricky. It was found on a ranch adjacent to the battlefield. And, yes, it was taken legally. I have an affidavit.
You gotta figure he just had to make a call. I think both are on target, as well as I think he was a bit anxious to succeed and sent Reno in before assessing the enemy force, then was unable to communicate with him as the Cheyenne were able to intercept messengers, a factor that he had normally had to face. Going back to your original thought, he did underestimate Crazy Horse, but he would have survived without splitting the force. Interesting stuff, as I lived a few miles from Beecher’s Island battlefield in Colorado and have a pretty good understanding of the battle for the West.