Look at the date on that article. It is from 2016. Tesla has sold a lot of cars and those cars have driven a lot of miles in the last year and a half. Tesla is certainly over a billion miles in Autopilot driving by now.

EDIT: Here is a Tweet from April 2016 saying 47m miles. [1] Your linked article is from November 2016 and shows 300m miles. That is 250m miles in 7 months. Projecting that forward with zero growth, Tesla would have accumulated roughly 950m miles in Autopilot usage. That isn't including the over 100,000 cars Tesla has sold since November of 2016 or the miles those cars have accumulated in that time. I stand by my statement that Teslas have recorded over a billion miles while using Autopilot.

[1] - https://twitter.com/Tesla/status/718845153318834176