Does it mean that he does not belong to the family (or to your group of closest people, friends, relatives, neighbours, etc.) or that he is someone you don't know or both? (Maybe it is because I'm thinking in Spanish, where familiar means both family and familiar, and that οἴκος meant both house and family, but family in the sense of the Latin familia, which included both relatives and servants).
Not really, άνθρωποι is implied. And can also mean "the people i know" based on context.
I answered "This isn't familiar" and it was counted wrong, the correct answer being "That isn't familiar"