Leading indicator of the (overall) tech job market is a better way to put it. The job postings in The Who Is Hiring? threads are a small sliver of the overall industry.

It's a fine sample though.

I’m not sure it’s representative. It has a higher proportion of startups and a lower proportion of established non-tech companies who still often make tech hires

Not the author but I would assume they mean that "Who is hiring" was indicative of the job market outside the HN thread itself, in a macro-perspective.

It is a pretty narrow segment of a much larger market, presumably.

"Startups quit hiring first," sounds like a leading indicator on the surface, but if you think about it that also means that they will stop hiring during smaller dips that later pick back up. Every big dip has to do through the smaller dip state, but since plenty of smaller dips recover it is still a martingale - unless it isn't, but if it isn't that's the cause of my surprise.

That's basically true of all "leading indicators", and is summarized in the popular joke that "Economists have predicted 8 of the last 5 recessions".

Yeah, there are literally thousands of tech companies that don’t use HN.