He's the exception. He had a lot of luck on his side. Of course he trained hard, but I'd say his circumstances played a big part in his silver medal.
Although many people know he won the trails to get into ADCC, he also lost one of the trials. He lost to a D1 wrestler who is also a blue/purple belt. At ADCC they didn't meet because the other wrestler was eliminated. Also I don't think Nicky Rod scored any points on anyone and won by ref decisions. That's not to say that isn't an accomplishment, because to do that well against experienced competitors is amazing, but the decisions easily could have gone another way. The last thing to keep in mind is that in the HW division people want to wrestle and will gas out quickly. Nicky used this to his advantage. The lighter weights don't gas out and won't even try to wrestle with him.
I'm not saying wrestlers can't win, but the idea that you could take a D1 wrestler and give him 2 years of BJJ and have him winning ADCC doesn't seem to be true. In Nick's division there was a high level D1 wrestler with a few years of BJJ and he lost.