In this situation in a built-up area, are you obliged to allow the bus join traffic if the driver signals such an intention?

Yes, in a built-up area, a driver is obliged to make it easier for a bus to join the flow of traffic. According to Art. 18 para. 1 of the Road Traffic Act, you must slow down or, if necessary, stop to allow the bus to pull out from the bus stop. In the video, we see exactly this situation: the bus is indicating its intention to join the traffic, and we are in a built-up area.