Then they can just skip insurance. Most people don't need it, especially if they already have a lot of money (which is what FIRE means)
Even on a pretty good Kaiser plan, we're paying $200+ per day in the hospital, etc. On a high-deductible, more. They say they have a $1M annual limit, but that they've never enforced it. I hope we never have to find out.
If you have an extra couple million dollars above and beyond your regular retirement fund you could self-insure your medical costs. But then you could just buy the health insurance.