Duolingo doesn't give context. Theoretically we would have already been talking about someone or would be pointing to someone. Regardless of whether we know who "he" is, the English sentence "he is cold today" and the Hawaiian sentence "anuanu ‘o ia i kēia lā" have the exact same ambiguity (well maybe the Hawaiian is a little more ambiguous since it can refer to a male or female).
since it can refer to 'he' or 'she' in theory it should also be able to refer to an 'it', right? "it is cold today" in the more ambiguous sense of 'something we were already talking about is cold today' should work right? not in the sense that the day is cold, the way "it is cold today" might normally be used in English?
The impersonal "It is cold" would not be Anuanu ʻo ia i kēia lā. No one would say it that way. It would just be said without a subject - Anuanu i kēia lā.
Understood, but is it grammatically incorrect? Suppose the "it" is the rain or the ocean. Is it i correct to say "anuanu 'o ia i keia la"?
If you are referring back to something like rain that was previously mentioned, you can use 'o ia, ia, or not use a subject. Like Spanish, understood subjects can be dropped if the context is clear.