Since the preface says writing about politicians can be Guru service, does it suggest this is a path to spiritual enlightenment?

No, the preface makes a very clear distinction. Chapter 0 states that considering great men like politicians or writers as one's worldly Gurus and writing their biographies is a form of 'worldly' service. However, it immediately clarifies that this type of service is 'limited by space and time' and 'does not yield spiritual fruits' because it is often rooted in self-interest. The text contrasts this sharply with the spiritually fruitful service of writing about saints, and especially the ultimate service of writing and studying the life of one's own Sadguru, which is said to grant the fruit of salvation.