On Mount Tamuke above Kokura stands a stone about four and a half metres high, erected in 1654 — nine years after Musashi's death — by 宮本伊織 Miyamoto Iori, his adopted son, by then chief retainer of the Ogasawara.
It is written in Chinese, in the formal register such things used. It gives Musashi's ancestry, his method, his fights — the Yoshioka, Ganryū — his character, and his death.
Everything downstream of it is downstream of it. The Hyōhō Senshi Denki around 1700, the Nitenki, the Edo literature, Yoshikawa Eiji's newspaper serial of 1935, the films, the manga, the business books, and the book you are holding.
It is not a neutral source. Iori was the son, was a senior official, and was establishing a lineage. Monuments do not record; they assert. Its account of the Yoshioka is the account a son would give.
And it is not worthless, which is the mistake the debunking literature makes. It was raised in the province where Musashi had lived, by a man who knew him, in front of people who had known him — including men who had been at Shimabara. A monument that is publicly wrong about a recently dead local figure gets laughed at. There is a limit on how far it can invent, and that limit is other people's memory.
An interested source, close to the events, publicly accountable. Which is the best kind of source this subject has — and the reader should notice that the best available evidence for the most famous swordsman in history is a gravestone put up by his son.