The programme makes the case that the Indians fought on horseback, in the main and that fact gave them the advantage over static military deployments.
It seems that the speed of the mounted attack and sheer numbers overwhelmed the poor fire rate of the dismounted soldiers, who were reduced in numbers by 25% due to every fourth man marshalling the horses (according to the programme). The men panicked and ran.
I think you may be misinterpreting what Scott said. Could you cite the time location in the video where Scott talks about it so I can look at it?
Yes, there were Indians on horseback during parts of the fight. They rode their horses into battle and used them to change positions (I suspect that's what he means if he talks about "manuever"). There were boys riding around on horseback far back from the front lines. There were a few individuals making brave runs in front of the soldiers.
The Indians likely mounted their horses at the end when the soldiers at Calhoun began to break and run. This is when cavalry can be effective: when foot soldiers are retreating in panic.
The pattern of Indian cartridges showed them grouped in limited locations along ridge lines and below hilltops where the Indians could lie down and fire from cover. They are not scattered around the battlefield as they would be if they were riding about firing from horseback. There's a tight group of cartridges down slope of Calhoun Hill that's been dubbed "Henryville". Do you think a couple of dozen Indians were crowded together on horseback firing from one spot?
We also have abundant evidence of how the Indians fought from the Reno-Benteen fight. They got behind ridges and fired from long distance, just like they did against Custer.
If you don't believe me, maybe you'll believe Doug Scott himself in the book he co-authored with Richard Fox, "Archaeological Insights into The Custer Battle" (all emphases are mine):
"From their
positions under cover, and intially at a distance from the soldiers, the Indian fire began to take its toll. (p 117)
"They
took positions close to Calhoun's line and poured in intense fire into his men." (p 177) [This was Henryville]
[Northeast of Custer Hill:] "
This position provided some cover to the Indian attackers as they fired into the knot of men on Last Stand Hill." ( p 119)
To my mind, it likely went like this: an extended period of static fighting with the Indians on foot and well-hidden behind ridges. The soldiers gradually weakened from this fire being delivered from several directions. As the soldier fire slackened, the Indians moved in to closer positions and poured in an even more devastating fire. At the end, seeing that the soldier fire had dwindled and perhaps seeing them start to break and run, the Indians rushed in on foot and horseback to finish them off.
I think the above description is pretty consistent with what Doug Scott believes. It's when you start trying to get into finer details about events that things start getting more and more conjectural.