Cargo doesn't care about the inconvenience of stopping for gas, and that allows cargo airlines to load the planes heavier (remember that much of a passenger aircraft is filled with air, not cargo). Anchorage is also within a 9-10 hour flight of almost the entire developed world, and massively overbuilt from the days when trans-Pacific aircraft had to stop there due to range limitations.

Yup - Anchorage International is the 2nd busiest cargo hub in the US (after Memphis).

Refueling isn't necessary (there are daily nonstop flights from Washington, DC to Beijing, for example) but having an intermediate stop may allow them to carry more cargo on each flight, as air freight is probably weight constrained rather than volume constrained, and less fuel would mean more weight left for cargo. (I could be wrong on that, of course!)

Looks like you're right: http://www.adn.com/article/20130630/fedex-cut-trans-pacific-...

It also looks as if the FedEx was using planes that didn't necessarily have as much range as typical passenger aircraft today.