Departure time from hotel depends on how you are getting to NRT. If using the Narita Express train, plan at least an hour in transit. For the limousine bus, a little more. If the hotel is not near a major station or isn't a pickup point for the bus, allow more time to get there. Then, 2 hours at the airport to find the right check-in counter, pass through the queue, check bags (if any,) do last minute shopping (if any,) clear security and make the way out to the gate early enough to meet airline minimums.
All of that could add up to close to 4 hours, so I suspect Lonely Planet was figuring the time from hotel check-out to arrival at the gate at least 45 minutes before flight time, which is what most international airlines require for economy passengers.