This, as they explain, is a chart of the Cyclically Adjusted price/earnings ratio (CAPE), formulated by economist Robert Shiller which is now very near its 1929 peak, itself only exceeded just before the dotcom crash of 2000.
Now, of course charts do not prove anything, and I have track record of saying markets are over valued. But then, that's usually when they are because fundamentals are being misvalued. And right now that is definitely the case, in my opinion.
I have a strong suspicion this will end in tears.