Isn’t this basically just what my comment about the edge of the knowable was
No. Not even close.
We know what obscenity is. A court will tell you if something is obscene. End of story.
The problem with the SC quote is, that it is at odds with the rule of law. The meaning of the law must be known. It can't be whatever some judge feels like. US courts use so-called tests to determine - with as much objectivity as possible - if something is meant by a statute or not. Currently, the "Miller test" is used for obscenity.
No true Scotsman is not an illustration of the edge of the knowable but of irrationality.