That doesn't check if () is true or false. Change it to ($x) or @a and you'll get exactly the same result. That check if = is true or false, which check the number of elements on the **RHS** of =.
That passage is poorly worded, leading to people like you quoting it as it says something other than it says. When it says (), when it says "()", it's actually referring to "the value returned by () in scalar context, which is the same thing as saying undef. () is not a value. It shouldn't list one of the many many operator that can return something false in scalar context.
