It returns a scalar, which isn't quite right, but since this is working on 2D Points, you can probably just treat it as a scalar, because the cross product of two 2D points is (0, 0, someZ).

However, often it is interesting to evaluate the cross product of two vectors assuming that the 2D vectors are extended to 3D by setting their z-coordinate to zero. This is the same as working with 3D vectors on the xy-plane.

If you extend the vectors that way and calculate the cross product of such an extended vector pair you'll notice that only the z-component has a meaningful value: x and y will always be zero.