Actually "chini mithi hai" is correct in casual hindi but when we use proper hindi I mean in grammatical sense then it is "hoti hai"
HOPE YOU ARE NOW SATISFIED!!
The course is trying to teach you the distinction between the general statement 'Sugar is sweet'/चीनी मीठी होती है (meaning all the sugar in the word is sweet) and the statement 'The sugar is sweet'/चीनी मीठी है referring to some particular sugar.