Why does "저" mean That if its "I" with the subjectiv particle? (If you call it like that)
Has it the same translation?
Not sure I understand your question.
저 can mean "I" (in its polite form).
저 can also mean "that (over there)".
Two different usages. Context tells you what it means.
Yes. The over there part is an additional nuance that exists in this specific form of the Korean that. As such nuance is lacking in the English that, its up to your own appreciation (and the course creators' decision in the context of Duolingo) if one should include it explicitly when translating into English.
I guess I'm not a tree. At least we've established that. (Still pondering my identity due to the whole "I am an apple." and "Who am I?" phrases we were taught)
저 can't be "I" here because there is no subject indication such as 은, 이 or 가 is next to 저