If you read the various reports of what was found in the village, you'll see there was an unusually large amount of abandoned material. Even large quanities of meat were left behind. One theory is that the travois were needed for the dead and wounded.
I do agree with this.
It does bring up another problem......There were plenty of tepee poles left at the village that could be used for travois, and they were said to have 20,000 horses available.
I still do agree with what you said, it just might mean they didn't have as many horses as some GUESSED.
I would think that a normal "lodge" pony would pull a half-dozen poles, with baggage on top of that.
With a travois, you only want two poles, so you have a lot of excess poles.
Probably can't put poles or luggage on all the ponies a family might have available (and most families probably only had the minimum horses required, anyway...only "rich" families would have a lot of extra horses). Ponies that are "packed" need to be ridden or led by a family member, not running free like the herd.
Just some guesses from watching/reading of such things...
No matter how good you are, you have to get there first...
When the average family moved camp, one horse carried two poles and the the tipi, one horse carried 14-18 tipi poles (7-8 each side), one horse carried wearing apparel and battle and hunting equipment, one horse carried stone tools, cooking untensils, etc. and one horse carried meat and food and probably grandma. Add to this the other old folks who didn't walk, the children too young to walk, those who simply didn't care to walk, the warrior escort that was always mounted, the larger ceremonial lodges that required extra horses, breeding and young stock that wasn't broke to ride or trail, etc, etc.
Absolutely true that time would be the first consideration. In this case they needed to move out knowing Terry was coming up, but not not so quickly they had to grab things and run. And yes the warriors were blocking Terry.
I'm going to quote Bradley just because this is one of my all time favorite quotes...
From subsequent examination of the ground I am convinced that there were not less than a thousand...with plenty more to co-operate with them, and not only would they have easily defeated the cavalry, but they would have given our whole command a desperate fight had we advanced that evening another mile. Their village was retreating, and they were there to cover it, and it was only for lack of an hour or two more of daylight that we did not come upon them in force and prove once more the terrific gallantly with which they can fight under such an incitement as the salvation of their all.
God I wish Bradley had lived through the conflicts. He would have left us a treasure trove of descriptions.