Why is_wp_error() is not returning false even there’s no defined error

The function is_wp_error checks if the given var is an instance of WP_Error class (source code as WP 4.2.2):

function is_wp_error( $thing ) {
    return ( $thing instanceof WP_Error );
}

As you can see, if the given variable is a instance of WP_Error class, the function returns true, even if the object is empty. Your $error variable is a WP_Error instance, empty but a WP_Error instance, so it returns true. You could do something:

if ( is_wp_error( $error ) && ! empty( $error->errors ) )

Or initiate WP_Error only on certains conditions (write here, not tested):

$errors = array();

if ( empty( $comment_content ) ) {
    $errors['comment_empty'] = esc_html__("Comment field can't be empty.");
}
if ( strlen( $comment_content ) < 30 ) {
    $errors['comment_short'] = esc_html__("Your comment is too short. Write down at least 30 characters.");
}

if( ! empty( $errors ) ) {

    $wp_error = new WP_Error();
    foreach ( $errors as $code => $message ) {
        $wp_error->add( $code, $message );
    }

}

if ( is_wp_error( $wp_error ) ) {
}