they're referring to how an image will get perceived if its flattened in a photo v. experienced with binocular vision. drawing from life is taking 3D and making it 2D. drawing from a photo is taking 2D and keeping it 2D. also it did occur to me a photo will likely struggle to reproduce color as accurately as it occurs in real life. i mean just yesterday i pulled over to take a photo of the rising sun lighting up the underside of mammatus clouds and my iphone couldn't do it justice. i don't think the claim is silly; i was an art major at one point and went through this very discussion in my life drawing class.