Does Chapter 18 suggest that a Guru's teachings given to one person should be kept secret?

No, Chapter 18 explicitly refutes this idea, calling it a "merely imaginary and a meaningless fuss." The text argues that not only direct instructions but even spiritual guidance received in dreams should be shared with everyone for their collective enlightenment. To substantiate this point, Chapter 18 provides the example of the sage Budhakaushika, who narrated his dream-vision of the Ram Raksha initiation to all. The chapter concludes this point by comparing the Guru to monsoon rain clouds that shower the water of self-bliss on everyone without distinction.