That's what they would do if they're /r/PICS ("Posts Illustrating Community Sentiment").
Reddit admins have said they won't allow subs to become NSFW if they aren't actually NSFW. But "Posts Illustrating Community Sentiment" can be anything, even outright porn. And of course, the good moderators at /r/PICS would never allow outright porn without properly marking their subreddit as NSFW...
Which makes it impossible for every third party app even the ones paying the ridiculous API fees, to display content as everything NSFW is only available on Reddits own webpage or App. (Or at least that’s what I read, but I couldn’t find the post anymore, so if anyone happens to have a source…)