They admit it's contrived, but this isn't very convincing

    func printSum(a, b string) error {
        x, err := strconv.Atoi(a)
        if err != nil {
            return fmt.Errorf("invalid integer: %q", a)
        }
        y, err := strconv.Atoi(b)
        if err != nil {
            return fmt.Errorf("invalid integer: %q", b)
        }
        fmt.Println("result:", x + y)
        return nil
    }

It's not adding anything that the Atoi function couldn't have reported. That's a perfect case for blindly passing an error up the stack.