Tbh shrinking the image is probably the cheapest operation you can do that still lets every pixel influence the result. It’s just the average of all pixels, after suitable color conversion.
The author of the article seems to assume there is no color conversion (e.g., the resizing of the image is done with sRGB-encoded values rather than converting them to linear first). Which is a stupid way to do it but I'd believe most handwritten routines are just that.