Are you obliged in this situation to slow down in the built-up area to allow the bus driver to join the traffic?

Yes. According to Art. 18 para. 1 of the Road Traffic Act, a driver approaching a bus stop in a built-up area is obliged to reduce speed. If the bus signals its intention to join the flow of traffic, you must enable it to do so, and if necessary, even stop your vehicle. This is a regulation designed to facilitate public transport operations.