That's not really even close to true anymore... There have been a few studies that have attempted to update the Trinity study, like this one that gives an 89% chance of success even at a 3% SWR:

https://www.financialplanningassociation.org/article/journal...

People in the FIRE communities are routinely discussing 3.5% and 3% SWR's, and see 4% as risky.

Of course, it's easy to look at the last ten years and nudge the number up.

Retiring in 40s is a pretty big decision. It's not impossible to re-enter the professional workforce in your late-40s or 50s, if things don't work out, but it will almost certainly not be easy.

Is it even possible to access US pension savings in your 30s or 40s?

In the UK tax advantaged pension accounts cannot be accessed until you're 55 (and 58 for my generation)

Yes, it's possible.

https://www.investopedia.com/terms/s/sepp.asp

These people are not conditioning their withdrawals on valuations. If you retired in 1978, yeah, 6% was probably fine. In 2000 you’d likely already be beyond recovery even today.