In this context, it's more like "stranger" than "strange person". In fact, "strange person" does not exist in Vietnamese but one would say "tính cách lạ" (strange characteristic) or something similar.
But another example given in the same lesson for me was 'anh ấy là một người rất lạ' - so would that be unusual to say?
Am I correct in thinking that the word "lạ" has a closer meaning in Vietnamese to "unfamiliar" than to the normal translation of "strange = weird"? That would make "stranger" and "strange (unfamiliar) person" more understandable.