If Sai Baba himself said 'I myself shall write my own story!', doesn't that render Hemadpant's authorship a fiction and undermine the book's authenticity?

This statement highlights a spiritual concept rather than a literal falsehood. It doesn't invalidate Hemadpant's role but redefines it as that of a divinely inspired instrument. In Chapter 2, Baba's words are presented as a promise: if the author surrenders his ego, Baba will provide full assistance. The text clarifies this dynamic: "Work started with this thought—be it hearing, reflecting, or writing— Is accomplished by the One to whom it belongs; he (the author) is only made a pretext." This indicates that the authenticity of the work is actually enhanced, as it is portrayed as flowing from the divine source itself, through a devoted individual who has successfully dissolved his ego. It's a testament to the power of grace, not a claim of false authorship.