No, I said "wider shots" not "wide lenses"...
There are two rules regarding diffusion filters which are somewhat contradictory:
#1 Wider-angle lenses need more diffusion than longer lenses to match sharpness.
#2 Wider shots need less diffusion than close-ups because we expect to see more fine detail in wide shots and don't need to see every pore and flaw in a close-up as long as the eyes look sharp.
The first rule is an objective one about matching sharpness and the second is a subjective one regarding viewer perception. The confusion comes from the fact that wide shots tend to be shot on wider-angle lenses and close-ups tend to be shot on longer lenses. But the reason we use heavier diffusion on close-ups of faces is because our eyes don't demand the same level of detail that we crave when viewing a wide shot of a landscape let's say. We look at a landscape shot and expect to see final detail in leaves of trees, blades of grass, the texture of brick walls, etc. but when we look at a face, an excessive amount of detail and texture in skin, for example, can be unwelcome or at least unnecessary -- we mainly expect things like eyes and eyelashes to be sharp, and strands of hair, but we often don't like seeing too much texture in skin.