You seem to have skimmed the post.
in a hypothetical scenario where he earns 5000€ from his normal job, has a mortgage, a car payment, utilities and food etc, he saves 500 a month (10%).
He has a job. A good job. He is able to save as well, from the job alone, like most people with a decent job. Unexpected events often happen - regardless of whether you have a good job, additional income, or no job. Personally, in that situation, I would rather have a good job and additional income sources - but you do you.
12x15x500 = 90000€
An extra odd side job stacking shelves at the local supermarket or whatever = Only God knows how much more one can add to the pot.
The eventual %APR revenues & profits from these savings, investments and whatnot = An even harder & wilder unknown.
And all this assuming one can conserve the same well paid jobs during the 15 years and if one is some sort of almighty & omniscient wise investor to make the best and infallible decisions about where to put our money without any potential setback & losses and without have had to spend a single penny of that in anything at all.
So again. How one can expect to accumulate at the end of 15 years the necessary do$h to posses: