From what I see, this is a rather reputable guy when it comes to crunching historical financial data. Basically he is saying that “less risk (which is not volatility), less return” also works within the same asset class if we look at different historical periods.
I have seen some pictures illustrating the same thing for bonds. ChatGPT had friendly informed me that during the 17th century, the Dutch Republic had a refinancing of their outstanding debt with an interest rate of 5 percent in 1640, followed by a conversion to 4 percent in 1665. The coupon payments were done essentially in precious metals, not paper.