It appears that, although registration is disabled, social sign-in is still possible and allows for account creation (even if registration is disabled). For example, Twitter - Social Sign In - Invision Community. Maybe turning those off will put us in a walled garden 🤔
I actually never bothered to click on that and simply assumed that it does the same as selecting the image 😅
Yes, this has been known for a long time. You wouldn't believe who reported it first 😄