Hmm. I think data showing hiring vs time for companies would be more relevant than average tenure of an employee.
i.e. The faster a company grows, the less likely/harder it is to maintain culture and enforce hiring/recruiting standards - I can see how that may lead to shitty interview questions.
At that growth rate, you'd expect the tenure numbers to be dominated by hiring rather than turnover, a point the article misses completely.
(Per their annual reports, Amazon had 88,400 employees on 12/31/2012, 56,200 on 12/31/2011, and 33,700 on 12/31/2010)