I'm in the core Millennial range typically mentioned in articles like this -- I graduated college in 2008.
On an $18k income, you would need to have $36k by 35 according to that rule. Let's assume I started saving at 25 (a few years out from college) and let's say I plonked my money into a high yield savings account, which is frankly far more likely for someone living right above poverty than investing it into index funds. For 10 years, I would have to put about $3.2k a year into savings, leaving me at annual spending of $14.8k -- very, very close to the federal poverty line. This assumes that you don't need to do something like spend $2k on dental surgery, thereby depleting over half a year's savings in one go.
If I had put it into index funds, I would have needed to save about $1.8k a year to have $36k by 35. That is more doable (leaves you with $16k to live on), but again: When your entire asset pool is $36k, you typically don't feel like risking it in the stock market. If you decided to do the sensible thing, and build up a good emergency fund and then invest (say $9k in an emergency fund, the rest in stocks), you'd be somewhere in between those two levels. (I'm too lazy to do the math right now.)
In grad school, getting a degree that has given me some real career opportunity, I lived on $18k in a HCOLA for six years. I paid my bills, and I paid off the interest that accumulated on my one private student loan (the others were all, blessedly, subsidized loans). I shared a big house with other people, and I didn't have a car. I was in good health, so I lived decently comfortably. I always had a thousand or two in savings, but I was not looking towards retirement. It definitely felt like a joke to me -- how was I to save the huge amounts I supposedly needed, when I'd be living even closer to the bone to do so? I did the calculators and they made me actually straight up cry. I felt like I could never get ahead, because every time I saved anything something else came along to take it away (hi that $2k dental surgery). I read personal finance blogs, and I tried, but every time -- boom. There went my hard savings. It was frustrating, and I felt like a failure. I desperately wanted to feel financially secure, to not worry anymore, and it was always out of reach. (And again, this is as a grad student; I knew it would change some day. Imagine not having that security backing you up.)
So, yeah, I get it when people get angry that they're told they should take their just-above-poverty incomes and move to a poverty-line life in order to meet some arbitrary rule. In 2010, 1 in 4 US households made under $25k a year. That's households, not just individuals. That's a lot of people who are already in a tight spot, and saving for retirement makes that feel even tighter. Should people still save, and try to raise their income? Gosh, yes. There is so much power in frugality and financial security. Do we still have it better here in the US, even at poverty level, than many people in the rest of the world? Gosh, yes. But I get why people are pissed at "rules" like that when so much already has to go to rent and making sure the toddler eats. When buying a new pair of shoes at Target to replace a broken pair is a decision to agonize over, being told that you're fucking it up makes you hurt, and then it can make you mad. Because whether to buy a $20 pair of shoes to replace ones with holes in the soles should not be an agonizing decision for anyone.