It is possible but not required. The original prompt uses kēia lā as the subject. So the word i is not required. Adding i would change it slightly to "It is hot today", instead of "Today is hot".
"Wela keia la" I agree with you. Their answer is *Today is hot." DL answered with"Today." When I entered "This day is hot" I was marked wrong. Otherwise, how would one say "this day is hot"? Imagine a group of hikers considering the next few days. Which day is cold and which day is hot? Ans: This day is hot.