Translation:Circles and triangles are not the same.
In this sentence "itu" is not used to refer to a specific object or a specific noun.
"itu" is used here to refer to generic nouns (or objects, or activity), to refer to all members of the noun group.
In this sentence it's used to refer to "all circles" and "all triangles".
It's used in the same way as making a general statement.
Air itu basah = Water is wet.
Es itu dingin = Ice is cold.
Kucing itu binatang = Cats are animals (A cat is an animal, the cat is an animal).
Berenang itu sehat = Swimming is healthy.
I thought that might be the case. This sort of construction is quite typical in Polish and Ukrainian (required in many cases), but I think it is the first time I have seen it in Indonesian. It is so common in Polish and Ukrainian that they teach students that the word for "that" or "this" is a sort of form of the verb "to be."
Actually, now that I think of it, I would have expected "adalah" or something like that. Can "itu" and "adalah" be substituted for each other in these instances?
Not a native speaker, but doing this course, it seems to me "adalah" is mostly used in definitions, to connect nouns together, and "tidak sama" is not a noun. And we actually did encounter "itu" being used this way throughout the course; this is definitely not the first instance. Maybe the first with two words connected by "dan" in the subject?
"A circle and a triangle are not the same" was rejected when it should be accepted. If "lingkaran" and "segitiga" are in plural form, should it read "lingkaran-lingkaran" and "segitiga-segitiga"? There are no indications that "lingkaran" and "segitiga" are in plural form in the Indonesian sentence.
Firstly, I agree with you that "A circle and a triangle are not the same" should probably be an acceptable answer. However, I'm afraid you have a misunderstanding of how plurals work in Indonesian. Reduplication can explcitly identify a noun as plural, however it's very common not to reduplicate when still talking about more than one of a thing.
For example. "Anjing makan daging" is perfectly translatable into "Dogs eat meat".
"Persegi punya empat sisi" is best translated as "SquareS have four sideS"