Not just porn! Since reddit only has one such tag you can apply to posts, many subreddits were using the NSFW tag for a wider variety of reasons such as spoilers, content which may contain unsettling material (aka "triggers"), and more!
The stoners on Reddit got screwed - many of the weed subs got the banhammer, and the rest got NSFW'd because spez wants Reddit as G-rated as Disney for the IPO.
To be fair, there's been a site wide spoiler tag for a few years now, but yeah before it was introduced people used NSFW for that if a community didn't allow actual NSFW stuff or whatever
It’s not all NSFW. The Synology community I followed went full NSFW in protest and I can still view their subreddit. I cannot view subreddits such as watchitfortheplot. So somehow Reddit is differentiating.
Well I know they forced some subreddits go back to SFW if it was obvious the NSFW was just to protest. Maybe that's the case for the subreddit you're looking at?
How does reddit tell the difference? Is it just nsfw communities not in the api or all nsfw posts? There is no discrepancy at least in the UI between NSFW gore and NSFW tits.