Even if we assume he used 500 gallons instead of 1000 (refueling when tanks were down to half-full), that's still a lot for a sail boat.

From my own perspective-- I have a 57hp diesel and average about 1.25 gallons/hr at about 7.5 knots. That's 6 miles per gallon or 666 gallons to motor 4000 miles. If we motored half the time and sailed half, it would be about 333 gallons. My last boat had a 30 hp diesel and used about a little more than half the fuel/mile of my current boat, which would be about 200 gallons to motor 2000 miles. (Strong headwinds could increase consumption/mile significantly, perhaps 10-25%).