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

No, Chapter 18 directly refutes the idea that a Guru's teachings should be kept secret, dismissing the notion as 'merely imaginary and a meaningless fuss.' The chapter argues that not only direct instructions but even teachings received in dreams should be shared with everyone for their enlightenment. To support this, it provides the example of the sage Budhakaushika, who narrated the dream-vision of his Ram Raksha initiation to all. This section makes it clear that the Guru's wisdom is meant to be shared widely, like rain clouds showering self-bliss.