Well considering the numbers the GP post quoted were the DJIA composite I think pointing out that their P/E ratio is in line with historical norms despite being up 600% in 25 years is fine in assessing whether there is some huge bubble in that number.
If you want to look at the broader market the NYSE composite index has a P/E of 21.1 which is a bit over the 18-20 range that most risk averse investors would be looking for, potentially pointing to the need for a correction, but again hardly pointing to some huge speculative bubble that is going to wreck the economy.