Your estimate seems sound to me IF they calculated the range by simply adding battery-only range to gas-only range, meaning that you don’t benefit from any regen (charging the battery from braking, etc.) during the gas-only portion of the trip. In the real world, this would mean driving a fully-charged car in ESave (gas only) mode until you run out of gas, then switching to electric mode until you run out of charge. (Which nobody would ever do.)
If I were calculating the total range, I’d approach it differently: I’d presume you made the entire trip in Hybrid mode, during which the battery is both drained from use AND charged from regen, which I believe would extend the total range quite a bit, in addition to more realistically reflecting how people would really drive.
If they calculated it my way, then we don’t have enough info to determine the efficiency of the gas engine by itself… but the number would definitely be lower than your estimate, as you’d need to subtract the number of miles gained from regen before you do the miles per gallon division, which can only lower the result.