Yeah, they're totally different genres and experiences. This is a history book, while Musashi is purely fiction based on a historical figure.
This examines Musashi as a real human being having to survive in a real world, the good and the bad based on different historical accounts (which it also examines and suggests which is more likely given the facts of the time), so there won't be anything about Otsu, Matahatchi, or Musashi fighting 70 men at once, but stuff about him maybe being manipulated into assassinating Sasaki Kojiro for political reasons, and conflicting reports of the duel, such as the participation of Musashi's disciples. This obviously opens up alternative as to why he and his life changed so much after that duel, and speaking of which, this book actually covers events and details of his life after his duel with Kojiro until his death, including his ambitions, his adopted sons, as well as the proliferation of his style.
It's refreshingly objective as well (and it'll address the sort of Musashi mythos that exists, starting with Yoshikawa's Musashi), and the while the author certainly respects Musashi and his legacy, he focuses more on the facts and less on Musashi's greatness; also, while he expresses which accounts and theories make most sense to him, he never pushes it so hard that it becomes unreasonable, and you're free to disagree with his point of view or simply take a wider stance.