$reverse_top_level works the opposite way according to Codex?

The default for reverse_top_level is null. Now let’s take a look at the function source:

if ( null === $r['reverse_top_level'] )
    $r['reverse_top_level'] = ( 'desc' == get_option('comment_order') );

As you can see, it takes the value from the comment_order option. And that is either desc or asc. If the value is desc, then it will be set to true. Now Walker_Comment, which extends Walker, retrieves the parsed arguments $r as last argument for paged_walk(). And that method is a method of the Walker class. Let’s look at its source. And there we can see the following:

if ( ! empty( $args[0]['reverse_top_level'] ) )
    $elements = array_reverse( $elements );
    // does other stuff here

As empty evaluates false equal to null or ! isset, this part won’t trigger. That’s it.