The article says "before painting." These walls are painted already.
Sounds like they used very cheap drywall to finish. If you’ve tried some cleaning products and there’s still a noticeable stain, touch up the area with paint. I keep paint containers stored with the paint I have used on the walls, for this purpose. If you don’t have any of the paint, but it’s a neutral builders paint and the house isn’t very old, ring up the builder and ask what paint they used. For touching up walls... I like to dry brush the paint and feather it out, so even if there’s been some color fading, it’s not noticeable.