Why does the preface seem to narrowly define 'Guru service' as just writing biographies? Doesn't this ignore other valid spiritual practices and paths?

The preface actually acknowledges multiple forms of service before arguing for the superiority of one specific type. Chapter 0 mentions that writing biographies of worldly figures like politicians or historians is a form of Guru service, but its fruits are worldly and not eternal. It also accepts that writing biographies of many saints yields spiritual fruit, though it may be a partial path. The author then proposes, based on his 'humble understanding,' that the most certain service for achieving salvation is to write and devotedly study the biography of one's own Sadguru. The text is not dismissing other paths but rather building a case for what it considers the most effective one.