You're right. Since I wrote that post, I found out that manufacturers are now paying attention to the CAFE gains afforded by switching over to synthetic diff fluid for factory fills. I still think the CAFE gains are tiny, but enough to justify the premium paid for synthetic diff fluid over the life of the fluid.
Some manufacturers have switched to synthetic for transmission fluid, but considering the premium to pay for 12 qts of (semi-) synthetic (as opposed to 2 qts for the diff fluid), I believe there's more at stake, such as longer life for the transmission, as well as adverse operating conditions, e.g., cold weather, towing.