That's not really true outside the US. And in fact it's not even true for the smaller cap stocks in the US. The Japanese stock market being the most cited foreign example of the opposite. Pretty much everywhere outside the US the stock market is very vulnerable. Odds are this will come to the US sooner or later.
They can QE forever to feed the demand, however it's on the whim of a few powerful people when and how some shortage will occur. Stocks grew 20% YoY with flat earnings in the last few years. Price discovery and market signals are dying, but they are dying for both directions up or down.
Many analysts expect asset prices and income inequality to be a major topic of debate this election and we don't know yet if Fed policy will be at the crosshairs. Either way social justice impulse will be stronger against the tech giants either thru cancel culture or outright riots.
Be clear: you are betting if you take Bogle's advice. You are going all-in. If you know what you are doing, great...but that is probably not the case.
It is "manipulated" to the same extent as anything else.
Of course as technology is getting faster and creating more value, and people are moving to cities, it's hard to get a correct valuation, but still gold seems undervalued compared to these other asset classes.
At this point I think it just depends on what the central banks want, as they took control over stock market prices with the low interest rates.