How are iterators functional language features? They're not unique to functional languages and they don't require function composition to implement. They're present in most popular imperative languages.
Even if it's not the only way to implement iterators, using closures which maintain state and yield values one after another seems to be a very popular way to do it.
In that case you're passing around a function, which matches how this chapter characterizes "functional programming".
In that case you're passing around a function, which matches how this chapter characterizes "functional programming".