The fight doctor made the comment a few years after the Spinks fight, he said that Spinks wasn't a great fighter, but a 6ft 2 200 pound strong man wailed away on Ali for 15 rounds, look at Ali in the 15th round of that fight, he was ready to go.
I think that was lack of conditioning as much as anything else but I think Spinks deserves credit for seeing the rope a dope as a golden opportunity being handed to him on a plate. Other fighters seemed to treat it with unnecessary caution.
Facing Ali is excellent. Have you read Facing Tyson? I believe its by a different author irrc, but it has the same core concept.