If it is one ticket then it can't be HBO, HBO is not combinable. Technically an agent could plug 2 non combinable tickets together "illegally" but Expedia doesn't do that. I would read this as a BA non HBO ticket going out, and an AB non HBO coming back. Air Berlin may give you the BA allowance but I can see the agent in Rhodes would simply enforce AB's rules. Luckily AB is more or less the same as BA, with the exception of hand luggage.

Before booking with Expedia, give BA a ring to see if they can book the ticket for you, doesn't always work, but it may do, and it gives you a lot more protection if something goes wrong.