He decoded everything, but he didn't actually open a car door. He still has to defeat the rolling code. It's not like you can add 1 to it and resend it. From the outside world, the next rolling code should appear random.
That's right there in the lede, "not as insecure as you think".
Is it hard to brute force? If so, it is still something that can be sniffed, though you can't unlock the car without first recording a genuine button press.