To be generous, there might just be a nation agnostic list of famous generals that are discussed in inner sphere military academies, and Stonewall Jackson could just be on that list.
For example, maybe the SLDF put together a list of past generals and famous engagements who their officer candidates are expected to study, and the Great Houses just kept that in their curriculum.
So Victor might know the reference to Stonewall Jackson and his battles and whatever lessons his instructors expected him to learn from them, without even knowing any of the greater historical context.
(Or maybe he summered on the Commonwealth world of Dixie)