It's a quote directly out of the byline. It is also, according to the author, the main thesis of his new book.

Is there some rule on here that the title of the submission must correspond exactly to the title of the linked article?

Yes, from the "Guidelines" link at the bottom of HN:

Otherwise please use the original title, unless it is misleading or linkbait.

https://news.ycombinator.com/newsguidelines.html

HN only allows less than 80 chars for the title. The LSE blog Twitter account gave it this title.

Not a good title, though, because it's a cherry-picked quote selected for baitiness. Such things have a pronounced (and in this case dismal) effect on resulting discussion.

Much better to shorten the original a bit, which we've done above. If it's necessary not to use the original (because it's misleading, linkbait, or impossible to shorten), the next best thing is to find an accurate, neutral phrase from the article.

If you want to say what you think is important about an article, please do so in a comment rather than using the title to express that. Then your view is on a level playing field with everyone else's.