Does Chapter 18 suggest that a Guru's teachings should be kept secret?

No, Chapter 18 directly refutes the idea that a Guru's teachings should be kept secret. The text dismisses the notion that what a Guru tells one person should not be told to others as "merely imaginary and a meaningless fuss." It provides the sage Budhakaushika as proof, who narrated his dream-vision of the Ram Raksha initiation to everyone for their enlightenment. Chapter 18 reinforces this by comparing the Guru to monsoon rain clouds that shower self-bliss, asking, "Why keep that bottled up? One should partake of it to one’s heart’s content and let others partake too." This clearly indicates that the Guru's wisdom is meant for all.