This is poor advice.

Based on your logic, if someone was to 'proceed with caution' regarding investing in AAPL when the P/E is >20 then you would have been proceeding with caution ever since June 2014 and most likely missed out on 750%+ gains.

Obsessing over historic P/E is ridiculous, given that the company and the wider tech industry has transformed drastically since 2006. There's no point using 2006 P/E data, it's out of date and not relevant.

Sure, there's an argument to suggest we are in somewhat of a bubble and P/E's are higher than they ever have been, but to apply some 'average P/E ratio based on the last 20+ years' is just silly.