I recently tried to redeem DC miles on Austrian Airlines between LHR-VIE. Austrian operates this route jointly with it's own aircraft and that of it's wholey owned subsidiary Austrian Arrows (also known as Tyrolean Airlines)

Diamond Club insisted that I could only redeem miles on the flights operated by Austrian itself and not Austrian Arrows. Is this correct?

I believe Austrian Arrows flights are considered members of Star Alliance. They certainly come up on the ANA tool.

Thanks in advance for any help.