The fact that BOH has an hourly bus link for the whole day to the train station is wonderful
I suggest you look at train arrival times and bus connections to get a passenger to the airport two hours before the early morning departures and revise this statement.
Also car hire companies are not open on some days when flights arrive later evening. Is it assumed there are no inbound visitors?
My first comment on bus/rail connection also applies to inbound flights for buses to the station and onward train departure times.