It’s not in paradise because it’s the point between the two. This is the same as saying reverse mountain is in the east blue.
It's not like saying that because Reverse Mountain isn't an island, it's a segment of the Red Line, nor was it a proper arc like Fish-Man island was, it was a five chapter stop point.
Is Fish-Man Island an island? Yes.
Is Fish-Man Island in the New World? No.
Is Fish-Man Island the last island the Straw Hats travel to before entering the New World? Yes.
It isn't complicated.