I didn't read the article, but: a GPS receiver must calculate/find both it's time and position to get a fix. So maybe by having the time already available really accurately it makes the job of finding position easier?
From my (very basic) understanding of GPS you need at minimum four satellites to calculate the time. If you had a local atomic clock in sync with the GPS satellites, you'd only need three satellites to get a position fix. It would (probably, maybe?) also speed up the time to first fix / time to a precise position fix.