You're missing the point in question. His memoirs (concerning Stuart's performance in the Gettysburg campaign) are at variance with the preponderance of the evidence presented in thousands of books and articles. Mosby was a partisan ranger who had no appreciable experience in combined-arms operations. Much of his time was spent in "behind the lines" small-unit actions. He was brilliant and effective; but he had virtually no experience in direct support of an infantry-heavy army on the move in enemy territory.

Granted that most Civil War historians never served in the Civil War, but their opinions and conclusions (at least those of the good ones) are based on years of painstaking research, exploring all available documents. Direct experience is not a prerequisite for an historian. Darn few historians directly experienced the Fourth Dynasty of ancient Egypt or the Punic Wars, but that does not disqualify them from being considered as expert Egyptologists or classical historians. Of course, there are those who for reasons known but to themselves are contemptuous of experts and professionals, but I take no notice of them.

Let me be clear that I am not categorically dismissing Mosby's memoirs in their entirety. When he writes about the things he personally saw and did, his work is pivotal. I consider his memoirs as a whole to be a Civil War classic, and I have always considered him to be a natural-born tactical genius. But, like all memoirs, you have to place his work in context -- doesn't matter if you're reading Robert E. Lee or Pvt. John Smith from Podunk, Mississippi. The job of the historian is to examine all pertinent sources, evaluate them, put them into context, and develop a thesis that can withstand professional scrutiny. Mosby's thesis on Stuart's performance at Gettysburg just doesn't rise to that level.