This is really a question about English. While "shape" is a noun, it is really being used as if it was an adjective in English in this case - think of it as "it's square" rather than "it's a square".
Is é an cruth ceart é is a valid sentence, but it doesn't mean the same thing as tá an cruth ceart air. If you were on a game show, and you had to choose between the square, the circle and the star to win the prize, the presenter could say Is é an cruth ceart é! when you picked "the right shape". But when I pick out a new triple-glazed window to replace the old energy-inefficient one, I need to pick a window that has the right shape to fit in the existing space - tá an cruth ceart air - "it is the right shape".
You could argue that "I have lots of money" and "I'm rich" mean the same thing, but that doesn't mean that you should translate tá lán airgid agam as "I'm rich".
Bear in mind that these exercises aren't designed to test your comprehension of Irish, they are designed to teach you Irish by exposing you to the way Irish sentences are structured. The tá X ar Y structure is very common in Irish, and it is worth teaching learners that it is often translated as "Y is X", and it's not just in sentences like tá brón air.
Interestingly enough, your suggested "The shape of it is correct", is an example of what you might call a "reverse genitive". To say "the shape of a ball" or "the shape of Seán", you would usually use the genitive in Irish - cruth liathróide or cruth Sheáin, and these could be translated back into English as "a ball's shape" or "Seán's shape". But you can't put a pronoun into the genitive, so both "the shape of it" and "it's shape" are translated using the possessive - a chruth - tá a chruth ceart. (I don't think you'd use the partitive genitive in this case).
This is an Irish to English exercise. Both tá an cruth ceart air and tá an cruth ceart aige can be expressed as "He has the right shape" in English, but they don't mean the same thing.
"He has the right shape so that he can fit through that gap" (air) versus "he has the right shape in his hand to solve that puzzle" (aige).
"It is the right shape" is a far more natural translation of this exercise, but people complain if they aren't allowed to translate sé as "he".