Well, considering that it doesn't seem to be an accurate statement, it shouldn't be so amazing that people don't "get" it.

By far, the largest shareholders in most publicly-traded firms are "institutional investors", but those are themselves in turn usually acting as middlemen managing mutual funds, most of which consist of ordinary folks' 401(k) plans and pensions.

What? "Doesn't seem to be an accurate statement"? What part? Those numbers are actually conservative. According to Yahoo Finance[0], it's actually 93% of the stock market is owned by the wealthiest 10% of American households. And the bottom 50% of Americans own ~1%. You "seem" to be mistaken and you're talking out of your ass.

[0] https://finance.yahoo.com/news/wealthiest-10-americans-own-9...

The article you're citing doesn't link to its sources, but seems to be talking about direct stock ownership by households, and not explicitly stating how it's accounting for investment funds.

I think it's accounting for indirect ownership as well. That would be very misleading and irresponsible reporting if it weren't.