alt Hacker NewsThe contents of the article make it clear that "touch wood" is basically synonymous with "knock on wood," making the knocking less important than the touching.
But is is important to mention that a ctrl-f on that page for "unpainted" is proof that you didn't spend much time looking.
Do you have reason to believe that the article knows something about the origin of this practice that no one else in the world knows? The claim is set in the past, not the present. And it has to be, since there is no present significance to unpainted wood in English-speaking cultures.