Reddit promoted “downvote to disagree” and this is a consequence of the userbase having migrated from there.
You can say whatever you want about how it should work, but the bottom line is that most people using vote-based systems are voting up and down based on if they agree with what’s posted. It hasn’t been “downvote for off-topic” for over a decade.
This guy wants to bring more redditors to the site, the people actively perpetuating downvote to disagree.