It is, but Thales is what this article attempts to prove (a). By the point you've proven Thales with isosceles (b), proving that a parallelogram with equal dia

It is, but Thales is what this article attempts to prove (a). By the point you've proven Thales with isosceles (b), proving that a parallelogram with equal diagonals is a rectangle thanks to Thales(b) in order to go forward along the Thales(a) proof is circular logic.