The disconnect is that $30k per year for people over there is living "like a poor person" and "barely scraping by". Over here on MMM it's viewed more as a pretty normal/happy life, especially if there's no mortgage/rent. Which means most of the $30k is discretionary spending.
I don't think it's completely crazy of them to point out to someone his age that if you need $30k per year as a single person, you might want to take into account that the future could hold a partner + children and those children turn into teenagers who'll have the same food & housing requirements as an adult at least for a couple of years.
And couldn't you go earn more money at that point?
It seems blatantly obvious to me that if you prepare for one level of spending, and you decide in the future to drastically up that level, you may not be able to support that new, higher level.
There's a bunch of future possibilities I could oversave for.
Maybe I'll want to become a pilot and buy a plane. Maybe I'll get divorced, split half my assets, then fall in love with a broke single mother with six kids.
Should I keep working until I can cover pretty much every contingency? (Let's call it, oh, 100MM or so.)
Or should I understand that I'm quitting now at a certain level and if things change, they change?
I wouldn't assume anyone who figures FIRE out is too dumb to realise expenses in the future might change. Likely they've considered that and are saving for it anyways.
See: Herbert Derp, the most frugal Mustachian, who spends ~5k/yr, but is saving to be able to spend 50k+.