Effective Java: Make constructors private or package-private

From Chapter 4 of Effective Java the following example is given:

public class Complex { 
    private final double re; 
    private final double im;

    private Complex(double re, double im) {
        this.re = re;
        this.im = im;
    }

    public static Complex valueOf(double re, double im) {
        return new Complex(re, im);
    }
    ... // Remainder unchanged
} 

[...] It is the most flexible because it allows the use of multiple package-private implementation classes. To its clients that reside outside its package, the immutable class is effectively final because it is impossible to extend a class that comes from another package and that lacks a public or protected constructor.

I understand that the class is effectively final,

but is there any actual advantage from not declaring it final?

Omitting this keyword when the class doesn't support extension seems like a less clear way to communicate how the API is meant to be used. Or was final only left out here to show the reader that non-extendable, non-final classes are possible?