Indeed. I switched jobs recently and wanted to provide notice but had to do so without a signed offer. I was thankfully confident enough that I'd be OK even if things fell through to go ahead with it, but in different circumstances I might not have been able to do it.

In many cases, it's OK to negotiate your start date a couple of weeks out from the offer agreement. I've gotten almost a month in both of my last offers.

Yes, and the point of this article is that Coinbase is rescinding employment for people who have accepted offers but haven't started yet. These people have already given notice at their current jobs and now won't have a new one to start. They are Screwed.

In California does a rescinded offer after quitting another job qualify someone for unemployment benefits? It's a small amount compared to tech salaries, but is helpful when between jobs.

I’m sure you could. You can collect unemployment even if you quit without a new job in many cases.

Well yes. But this is still the correct procedure to follow, even if it has some risk.