Intelligence is not a science, certainly not a natural science.
What you mean is economics is not a natural science.
While it's true that economics is not a natural science, that is trivial and not what they mean.
Every known fact in natural science was divined by the presentiment of somebody, before it was actually verified.
More subtle is the distinction of natural science into physic and metaphysic.
That's who told me about the natural science dept that overlaps my field.
Nor did natural science, philosophy, and history cease to develop fruitfully.
It highlights a range of styles, techniques and subjects in the natural science field.
It does not conflict with understanding natural science.
There will always be some things that will not be fully explained, but the same is true for natural science.
British Dictionary definitions for natural science
the sciences collectively that are involved in the study of the physical world and its phenomena, including biology, physics, chemistry, and geology, but excluding social sciences, abstract or theoretical sciences, such as mathematics, and applied sciences