This is translated “who the man was” but doesn’t “zu wem der Mann gehörte.” literally mean “to whom the man belonged”? Is this an idiomatic usage that I’m not familiar with?
Agreed, I would read it literally as “to whom the Man belonged” (and interpret it as something like “whom/which group/… the man was associated with”). I am not aware of any idiomatic expression myself (though not a native speaker), and in fact @morbrorper seems to have raised this back in January on Tatoeba too, with apparently German native speakers agreeing about the uncoupling from the “who the man was” translation/sentence:
With further comments there suggesting it rather seems to be the result from an original misunderstanding in the translations and the meaning as you’re stating is indeed correct.