Streams: "cannot convert Comparator to int" when using method reference

Suppose I have a stream of Strings:

Stream<String> stream = Stream.of("c","a","b");

This works OK:

stream.sorted(Comparator.reverseOrder())

But using alternative syntax doesn't:

stream.sorted(Comparator::reverseOrder)
                        ^
   Bad return type in method reference: cannot convert Comparator<T> to int

When I Ctrl+Left Click in my IDE both take me to the same static method returning a Comparator in Comparator class:

  public static <T extends Comparable<? super T>> Comparator<T> reverseOrder() {
        return Collections.reverseOrder();
    }

One book explains:

Comparator is a functional interface. This means that we can use method references or lambdas to implement it. The Comparator interface implements one method that takes two String parameters and returns an int . However, Comparator::reverseOrder doesn’t do that. It is a reference to a function that takes zero parameters and returns a Comparator . This is not compatible with the interface. This means that we have to use a method and not a method reference.

But I don't understand it.