Well not if there's a ground plane beneath, with 4 layers you can have signal traces on the outer layers and make the inner layers ground and every trace will have a good ground reference.

With a 2 layer board though, one layer's "copper pour" is the other layer's "ground plane"... Yet the article seems to advocate that you shouldn't use copper pours for 2 layer boards either? That's weird.

You want a ground opposite of the signal, next to it doesn’t count.

When you understand that the electrical field of your signal is in the fiberglass not in the copper, you’ll understand why.