I watched this also. The key take away for me is

He said something like "If  you invest in S&P when PE is high (probably higher than 22). In the next 10 years, your annualized return is just 2%."

The current cash yield is around 3.5%-4.5% depends on where you park it.

That means, when the market valuation is high, it doesn't make sense to buy it. 

You can hold the position you bought at much lower level. But commit fresh money to the market when the valuation is high doesn't do any good.