So, in summary:

"A hammer can be used for the following things:

1. To drive nails into another material

2. To 'hammer out' dents in sufficiently soft materials

3. As a weapon

4. As a doorstop

5. etc.

Because some people only use it to do (1) and (3), it isn't a tool for (2)."

P.S. I know the author acknowledges this in his own comment on the article.