This is all true, but you leave out the fact that it's hard for someone making say, 20K a year is going to have a hard time living off 3K a year while they save the rest of it (after taxes). This is easily possible if you live with your parents, take public transport, and eat your parents food. But saving enough for 25 times expenses isn't really that great in this scenario unless you plan on living with your parents for life.
Savings rate is great, but it is not as simple as you make it out to be. It's easier to save most of your money and still live a decent lifestyle when making big bucks, than it is to do it off minimum wage...
THAT is what the OP is getting at, and it is a valid point. IMO, your time to retirement will either be longer, or you will need to cut back drastically. I'm not so sure that the cut back drastically is sustainable long term. Even Jacob moved out of his RV and into a house...
I'd say OP's main concern is more about retirement age than having a borderline poverty income. He or she talked about a couple retiring in their 30s with a high income.
As OP's post indicates, that's not feasible on a middle class income. But as other posters have shown, if you keep your expenses low enough, you can set yourself up to retire at 45-50...and that's 15-20 years ahead of 80% of the population.
I am living proof that's not true.
My first year out of college, I started at 32k as a teacher. My wife, as a part time substitute, made maybe 18k (then got a job as a full time teacher the next year).
Our final pay was 44k/yr. each. That was after working 8 years, and getting Master's degrees. Our salary cost of living increases were frozen for several years due to the recession, so most of that increase was the boost from the extra education.
Nearly a decade of experience, plus Master's degrees, and we still didn't hit even 90k combined, let alone 100k.
But we hit the 7-figure mark, and FIRE'd by 30.
Lots of extra income from side-gigs and other paid optional work, combined with a high savings rate, and investment into assets (real estate) that allowed us to put in extra effort for extra returns (not passive, but compensated by giving higher returns).
If your primary job won't get you to FIRE as quick as you'd like, you need to find other strategies to boost yourself and decrease your time to FIRE.
We loved our jobs, otherwise we'd have gone and done something more lucrative. It was worth it to us to keep those jobs, but since we still wanted to FIRE quickly, we made up for that via other paid work and savings and investing.
It was hard work, no doubt. We didn't take summers off, like we could have as teachers. But we made it work. (In fact, we retired younger, and with more money, than the people in the article.) You can too! :)