Earlier quoted context omitted.
That 4% is empirically derived from data covering the 20th century and is appropriate for earlyish retirement. There's two reasons to be more conservative: 1) We may be in a low-yield world for a long time. 2) Those studies were based on earlier retirement, not on people leaving the workforce in their 20s and 30s, the way that some very lucky HN readers may do.
Although on the flip side, the number also assumes you have no flexibility and don't change your spending according to how much you have left. It's a tricky one to work out, and 4% is (as you say) definitely not a number you can easily use for retiring in your 30s.
Definitely. If you're willing to constrain consumption when required and work again if you have to, things change. Especially if you continue to work just enough in your professional field during early retirement that you can go back to earning an appreciable fraction of your previous earnings on a temporary basis if/when required.