Yes, the image of Miyamoto Musashi that is popular today, largely through the publications of early and mid-20th-century Japanese authors as well as a range of Western (non-academic) translators, is highly mythologized and exaggerated. The definitive study of the invention of Bushido and the incremental creation of the whole mythohistory of the samurai is Oleg Benesch, Inventing the Way of the Samurai: Nationalism, Internationalism, and Bushidō in Modern Japan (Oxford University Press, 2014), which is required reading for anyone interested in an evidence-based, scientifically researched, and verifiable account of how the idea of the samurai became so central to Japanese national identity.

By contrast, the available books about Miyamoto Musashi in English are universally by non-academic authors and are published because of the enormous demand for lively stories about the samurai that match the mythohistory.

The situation is not much better in Japan. The vast majority of publications about Musashi in Japanese are highly mythologized, printed in manga format for younger readers, or distributed for martial arts practitioners. If you look at the catalog of a major Japanese library like Waseda University under "Miyamoto Musashi," for example, most of the results (1650 items!) are non scholarly.

I do see a few exceptions, such as the prolific writer and professional historian Watanabe Daimon's book Miyamoto Musashi: nazo ōki shōgai o toku (2015) [translates to something like Solving the Mystery of His Many Lives]. The very fact that he has to title the book in this way warns his readers that the evidentiary base is thin, however. As the reliable historian Cameron "Cappy" Hurst notes in his book Armed martial arts of Japan: swordsmanship and archery (1998), "The basic facts of Musashi's life are disputed" (p. 51).

In short, Miyamoto Musashi does seem likely to have been a duelist who lived in the seventeenth century, and wrote the now iconic Gorin no sho [The book of five rings] around 1643.