I also did not down vote the poster; however I don't see anywhere in the guidelines that says down voting is not for disagreeing. Interestingly though the guidelines do explicitly state, "Resist complaining about being downmodded. It never does any good, and it makes boring reading."

For anyone looking for the citation you can see the full (and quite short) guidelines here.

https://news.ycombinator.com/newsguidelines.html