I agree with your overall premise. But I think you're probably downplaying Nog's BJJ a bit. Yes, his style was pretty basic. But many of the best BJJ players appear basic because they: (1) stick with fundamentals; and (2) don't give up position trying to be fancy. There certainly were better pure BJJ players than Nog, but he applied it to MMA better than almost anyone.
And I'd say that his BJJ was clearly better than Mir's. In my opinion, Mir's BJJ skills are a bit overrated. Mir was an absolute monster in terms of catching submissions in scrambles. And his submission skills were very slick. But his overall BJJ game wasn't anything to write home about. People tend to think "BJJ" and "submissions" are synonymous, but there's a lot more to it. When you think back on Mir's career, there are very few fights where he dominated via BJJ. He generally won striking matches or caught people with subs in scrambles. He wasn't someone who could pull guard, execute a sweep, slice through his opponent's guard, and set up a submission.
Nog got careless in a scramble, which was Mir's greatest strength, and the rest is history.