Except it's not

I just read this sentence in a Slate article on Wittgenstein:

"So, language is quicksand—except it’s not."

On the face of it, this doesn't make much sense. To use except, one should normally use a list first, and a single statement hardly qualifies as a list. The use of except can be made acceptable if we see the preceding statement instead as a kind of container of truth. The quoted sentence then says: 'So, that language is quicksand is completely true -- except...' The first part is then scalar and this also allows for an exception, in the same way that we can say "I liked everything about the show, except the finale".

So far so good, but what follows except in the quote above is not really an exception to the totality of truth of what preceded. It's a complete denial of what preceded. It may seem somewhat pointless to state something and then deny it right after, but the [Statement -- except pronoun + {contracted verb + negator / verb + contracted negator}] pattern is very common. Here are ten examples retrieved from the Corpus of Contemporary American English (COCA):

1. I mean reaching back to what EJ was just saying and people think he's a Columbia Harvard guy for whom everything's always been sort of easy... GEORGE-STEPHANOPOU# (Off-camera) And Hyde Park. EJ-DIONNE-1 " THE-W# Hyde Park.DEE-DEE-MYERS-1 " V# Yeah. GEORGE-WILL-1-ABC# (Off-camera) Because he is. DEE-DEE-MYERS-1 " V# Well, except that he's not.
2. Dj vu all over again. Except that it wasn't.
3. It was almost funny. Except that it wasn't.
4. Well maybe the whole crime decline was a myth. Except that it wasn't.
5. Neuroscientists told Schmorrow that it was impossible. Except that it wasn't.
6. Finally, it seemed everything was going to be great. Except that it wasn't.
7. Until he found himself on a winning streak, with a small crowd cheering him on, and the only thing to do was keep betting. Except that he didn't.
8. ... and she knew he would turn at once to watch and make sure she didn't brush the moisture off her eyelashes except that he didn't.
9. You can't keep going like this, Jon, you'll burn out. He heard it so many times that he almost believed it. He must be exhausted. Except that he wasn't.
10. He cruised through his workouts for NBA scouts and believed he had a good chance of being taken No. 1 in the draft by the Chicago Bulls. Except that he wasn't.

There's something of a conventionalized ironic understatement in this cliché: 'yes, everything you (or they, or even I) have been saying makes perfect sense -- apart from the small tiny problem that it's just not true'.

Update: I just realize that I searched for the pattern with that after except. Removing it from my search query shows the that-less version is rather more frequent than the that-containing version. Here's just one bonus example from COCA of this more succinct form:

There was no appeal, and that was that. Except it wasn't.