[racket] htdp/2e: on types and popular scripting languages

Near exercise 305, HtDP/2e says that "[in] ISL+ (... and in the currently
popular scripting languages) such an informal signature with a definite
meaning is acceptable on occasion; do not use it too often, however."

It refers to this definition.

; [List-of Attribute] or [List-of Xexpr.v2] -> Boolean
; is the given value a list of attributes?
(define (list-of-attributes? x)
  (cond
    [(empty? x) #true]
    [else (local ((define possible-attribute (first x)))
            (cons? possible-attribute))]))

Why "acceptable on occasion"? I don't get the message. What is it saying?
Thank you.
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