As the BKK-flight is operated in/out of Terminal C, they must have connected on EW to BHX.
BE would have been the only other option, but they are always at the C5x-bus gates.
EW to BHX operates out of Terminal A (Non-Schengen), so TWOV would have been possible.
Seems like EW did not inform Klüh at DUS, about that request and therefore they were not picked up at the ACFT.