Earlier quoted context omitted.
I wonder what makes them think the stock market is overvalued. Increasing inequality combined with market saturation and the current difficulty to start a competitive business means it makes perfect sense that stock prices are historically high. Combine with the fact that passive index investing has become the norm, and it seems like it will be a new normal.
It explains each of the indicators on the page. Click the "Market Overvaluation" button (albeit with a pretty poor UI and terrible URL support so I can't link it). It's basically the value of US companies on the stock market divided by the GDP, $27 trillion / $20 trillion = 135%. Although it makes you wonder about if all that stashed money overseas is having a significant impact on that.
It is my understanding that the value of a stock should be equal to present value of future cash flows. If those future cash flows are growing faster than the discount rate then a value higher than 100% of GDP is to be expected.