A graph, three paragraphs and a reference to Yuval Noah Harari? There is absolutely no substance here. Maybe something meta about lack of attention spans and not looking beyond the surface at claims.
Yes, there is some hand waving, but it conveys the general idea that current wisdom and circumstances have made investing in large sets of stocks all the time with no regard to the underlying fundamentals does put an upward pressure on prices.
No the s&p 500 is not "everything", it is one, albeit very popular, index. There are many others, but the underlying concepts are the same. Further, the companies are not selected for long term performance. Ignoring some finer details, the s&p 500 is the largest 500 US companies by market cap. Perhaps you are confusing this with the DJIA which is hand selected, though not selected for long term performance by any means; rather, it is selected in a way to represent the economic landscape - how well it achieves this is a matter of debate for sure.
Investing for retirement shifted from a company taking responsibility to employees taking responsibility with the advent of 401k and similar plans instead of defined benefit pensions. Over time it has been noted that picking individual investments tends to do worse than simply buy everything.
As workers pour money into these non-discriminating investments, hoards of people are blindly investing and skewing away from efficient markets. Everyone (scare quotes everyone) is just buying large swaths of everything all the time without regard to fundamentals.
It's all good as long as this continues, but a shift in enough people no longer believing this, will reduce demand and prices will go back down.
In other words, people just buy everything because there is no alternative. If enough stop doing that, prices will readjust downward because too many stocks are overvalued due to the current wisdom of buy regardless of price.
It's hard to take these grand career advice articles too seriously. Unless someone can prove they have been able to live thousands of times, then maybe it is worth taking seriously. Otherwise, just another data point to keep in mind but not much more.