This contention is actually trivial to dispel. As a late 2000's HN user, I can assure you there was no secret 2009 groundswell of jobs, and that there was no HN before 2008.
Without seriously digging into the data, 2001-ish (dot-bomb) was pretty much a nuclear winter for tech and 2008-or so was enough of a general downturn that I made the decision not to play my networking cards even though I didn't feel in a good place with my then-employer. Today doesn't seem great--certainly not quit a job/have a new job by Friday great for most much less work multiple "full-time" jobs like some people were boasting of a while back. Stuff is certainly not falling into your lap to the degree it was 2-3 years ago. You have to chase it and may or may not luck out.
2008-9 was pretty bad. When I got laid off I decided it was better to take myself off the market and go back to college a few months later to finish my degree (I had 2 more years of school left) than try to fight for a job right away.
I can't pull that trick off again. But at least right now I still have a job, and don't seem to be too much at risk of losing it for the moment (work at a consulting firm and developing some highly requested features that will make a lot of money at my current client).
My contention is that pronouncing doom for hiring in the software sector based on a graph that does not include any previous boom/bust cycles is, at best, misleading.
I actually didn't see that these data are HN-specific, but that only serves to reinforce my point in two ways: 1) HN is only a small sample of the overall industry, and 2) since HN hiring presumably skews disproportionately toward the startup/VC funded ecosystem, it's likely to be even harder hit by interest rate hikes and economic downturns.
What? The contention is that jobs had booms and busts before HN. Starting off the low base of 2008, of course the graph shows growth. It does not show the 80s, 90s, or the dot com boom, or the dot com bust.
