If you have not a lot of savings to live from, and your runway is therefore short, you'd be stupid to quid your day job and launch your startup: your small savings will quickly be used up and that'll be the end, because you won't be able to raise a round.
If, however, you have substantial savings to deploy so that you can get far enough to afford to pivot 1-2 times until you find product-market fit, then PG is right and taking the plunge is the right thing, regardless of business environment.
If your plan relies on customers, then it also depends on whether your product or service is aimed at reducing customer cost or has value in a different way.
To exemplify the above: I claim "You can/should start something like AirBnb in a bad economy, but not Google."