That’s how it is. Even on the largest porn instance (lemmynsfw.com), you need to check the box explicitly on the sign up page. (And on the second biggest I know of (pornlemmy.com) there’s a full page consent splash screen before even you can even reach the sign up page.
The thread starter is complaining about posts which should be marked as NSFW, but are not. I personally haven’t noticed any of that, but I mostly stick to “subscribed” views instead of “All”