The article probably right about the overvalued issue because his judgement is based on historic P/E value. If he is trying to judge by historic average P/E, then we should expect the historic average return, which I believe is impossible for the near future.
Then should we continue to invest on stock market? For long-term investors, I think we should. How to get this conclusion? I compared stock market return to bond return (remember we discussed the relative about bond interest and P/E?), with such a low return from bond, a lower return from stock market is acceptable.