So just live with your parents for 20 years, hope the market goes well, and you'll be a millionaire? Looking at postings around the area where the worker lives, an apartment is about 4x the cost of what's he's paying for both rent AND food (
https://essex.craigslist.co.uk/d/flats-housing-for-rent/sear...). That's not even accounting for unexpected medical expenses, trying to start a family, etc., and all the other expensive stuff in life.
I'm not even all that sympathetic to the worker in the article, but "just park all of your money in an index fund and you'll be a millionaire" is quite naive.