It's a 2 month old article by perhaps the most respected figure in valuation. He came up with 2750 for S&P at that time (market value was much lower, maybe he would be more pessimistic now). You can enter your own assumptions into his spreadseet and it will output a valuation.
Of course the price is what it is due to supply and demand but I don't think that's what most people mean when they say the price is 'not sensible'.
The stock prices are so high because stocks and real estate are the only investments that might provide any sort of real return, and we've been in that situation since bond returns crashed in the wake of the 2008 financial crisis. In that context, stock values are massively inflated (in terms of P/E) relative to what investors may have considered sensible before 2008.
Even academic detractors of the efficient market hypothesis agree that it is accurate on the macro-scale.