The author of this paper clearly doesn't understand the economics of stock buybacks. They are simply a tax efficient means of returning cash to shareholders, something Apple definitely has needed to do for a while.
In all seriousness though, 20 is not a low P/E. It's just that AMZN's 170 P/E is very, very high. Facebook and Google are in their 20s I believe.
If AAPL were trading at 10x or 12x earnings, that would be low, but still not insanely low.
Just to emphasise this point more: to first order, a buyback of $n of stock reduces Apple's market cap by $n. Apple would have hit a trillion much sooner had it not bought back so much stock.
This article is hilariously off-base.