Doesn’t the message you received basically say the graphene devs don’t want it discussed in /r/privacy ?
If I were involved in a project with any sort of following I wouldn’t want it discussed in a large, general, subreddit either. If it is, you either need to engage with people there to minimise any misinformation, or you just have to let people spread nonsense about your product.
That said, asking why /r/privacy exists when the devs of privacy-related projects don’t want to participate is a good question. The answer is, the mods are fief lords who would rather preside over a sham than nothing at all.
Honestly, I can’t think of any good reason to be a moderator of /r/privacy on reddit
Nextcloud really does feel old.