let vs cl-flet scope

scope.el

(defvar foo 2)

(defun return-foo()
  foo)

(defun just-call-return-foo()
  (return-foo)) 

(defadvice return-foo (around change-it activate) 
  "This let block redefines foo in the expansion of ad-do-it"
  (let ((foo 10))
    ad-do-it))

(defadvice just-call-return-foo (around change-baz activate) 
  "This flet block has no effect on the function just-call-return-foo"
  (cl-flet ((just-call-return-foo () "hi"))
    ad-do-it))

(return-foo) ;; returns 10
(just-call-return-foo) ;; returns 10, hoping for "hi"