Sure, fp in Lisp might not always be true (scotsman) fp. ;-)
But omitting fp in the book is not evidence that Ousterhout is ignorant of fp, and there is certainly evidence to the contrary.
The likely explanation, given that he's developed a number of systems from Sprite to Tcl/Tk to RAMCloud to HOMA, is that he is addressing the current practice of systems programming, which remains primarily imperative.
Fp with lisp is only a fraction of fp. I of course am talking more along the lines of pure fp which lisp is not.