Important to note that the book is an adaptation of content from Sussman and Hanson's Adventures in Advanced Symbolic Programming course[1]. My sense is that the 'Flexibility' angle is a bit of a misframing, making it sound more like a rough and tumble best practices book, rather than an elaboration on neglected idioms from symbolic programming that they feel deserve wider adoption, paired with blue skies ideas on how to make software robust.
[1] https://groups.csail.mit.edu/mac/users/gjs/6.945/