I think this issue and/or statement about NR's sobriety can be misleading for a couple of reasons.
First, I learned for the first time after this horrific murder that there's a
harm reduction sober-- better known as "California Sober" which seems to be defined as abstaining from heroin and meth, but drinking, smoking weed, and taking pills is allowed.
Second, NR met a writer in rehab several years before "Being Charlie" was released, and for about 5 years, they had been writing a script about life in rehab. Nick and his co-writer wanted their script to evolve into a TV series! IMO, I think NR and his co-writer friend hit a wall with everybody before he ever considered turning to Rob, most likely because he believed his only option to "sell" this venture to Rob would require him to be committed to sobriety. I think the question that remains is whether or not Rob shared Nick's definition of "sober."
IMO, Nick ultimately sold Rob on whatever he needed to hear. We know Rob already harbored much guilt for believing the "experts" and not his son about rehab. And we do know that Rob was promoting Nick and the film ("Being Charlie"), on the pretense that its screenwriter was in sobriety (the old fashion definition)-- living in a sober house, and it was Nick who soon confessed to a podcaster that "sobriety was a relative term," and he was living a lie....
(Appearing on a podcast a few weeks later, Nick would be more candid about his ambivalence toward the movie and the promotional efforts around it. Sobriety, he said, was a relative term. While he’d managed to get off meth and heroin, he was still drinking and smoking pot. “I said to [the film’s publicists], ‘Listen, I’m not in a position to do this. I’m not a quote unquote sober guy.’ I’m going to have to go on these talk shows. They said you have to do this. They want the whole father-son angle. It just goes to show you how disgusting it is.” But he finally relented. “It was uncomfortable, but I felt like it was part of the job.”)
"I'm not a quote unquote sober guy"