Does Chapter 18 suggest that a Guru's instructions should be kept secret from others?

No, Chapter 18 strongly refutes the idea that a Guru's instructions should be kept secret. The text describes this notion as a "merely imaginary and a meaningless fuss." It clarifies that not only direct instructions but even teachings received in dreams should be shared with all for their enlightenment. To prove this point, the chapter cites the example of the sage Budhakaushika, who narrated his dream-vision of the Ram Raksha initiation to everyone, reinforcing the idea that a Guru's wisdom is for universal benefit.