These are my thoughts exactly.
I could maybe buy 60 mpg, at 5000 feet, for a short run, downhill, with a tailwind. Even though elevation plays a part, it's pretty tough to go almost 800 miles and stay at 5000 feet, and not do any stop and go, and never go up any long grades, etc.
The other consideration here is that the claim is for 64 mpg AVERAGE over 700+ miles. We all know that average means there are times where you get a little more and times when you get a little less. In my own experience, I've reached as high as 50 with a few lows of 40-42 for the overall 2000 mile average of 47. To average 64 means the peak would have to be maybe as high as 67 or so.
I would be very interested to see the mileage broken down by fill-up... what was the mileage for each leg of the trip?
EDIT: Actually, I would be very interested to see the data on just one fill-up. For any given leg of the trip, what were the gallons used and miles ridden?