Excellent point that you make, and something that I hadn't considered. I think the author was saying that the Indian ponies were more mobile, which is not exactly the same thing. Mobility takes-in a range of characteristics: speed, agility, range, endurance, etc. Guderian made the point that the engines of his tanks were as important as it's main armament.
I took it as meaning that a light cavalry could live off the land and could, at a marathon pace, travel a longer distance than a grain-fed cavalry could. A grain-fed cavalry would need to operate like a multistage rocket, starting out big and leaving most of your initial quantity of horses behind with enough grain to RTB.The kicker being that the grain-fed cavalry had some capability to operate in the winter, and the grass-fed cavalry did not.