From HN guidelines:

> Please don't comment about the voting on comments. It never does any good, and it makes boring reading.

and yes, downvoting is a perfectly fine disagree button, what else do you think it is

It makes boring reading because the discussion, for those who have cared about it in the past, remains the same.

Anyway, downvote = “This doesn’t belong here.” or “This doesn’t attempt to contribute to the discussion.”

If everyone here downvoted each time they disagreed, there would be few comments alive…and little discussion.

The reply button is the disagree button.

Otherwise, for comments of any length, it isn’t even clear what’s being disagreed with!