It seems pretty obvious when you consider that outer space is mostly just a gigantic void without substance.
I think you're confusing two different ideas of "void". You're talking about outer space in general being mostly space but the article is talking about a contrast in the density of matter distributed in different areas of the universe, using the word "void" to refer to an area of low density.
I've read some of Stephen Hawking's works and it seems clear to me that you're muddled in your thinking.