The text suggests the Laghu Guru Charitra is more authentic than Saraswati Gangadhar's Brihad Guru Charitra. Can you explain why?

This appears to be a misunderstanding, as the text in Chapter 0 does not suggest that the Laghu Guru Charitra is more authentic than the Brihad Guru Charitra. The preface clarifies that the Laghu Guru Charitra is a separate, distinct work that "appears to be much older than the Thorale Guru Charitra" (another name for the Brihad Guru Charitra by Saraswati Gangadhar). It notes that the protagonist is the same and provides a specific Samadhi date from this older text. However, as Chapter 0 presents it, this information serves to differentiate the two works and provide historical context, not to make a judgment on their relative authenticity.