"Downvotes are healthy and important for the ecosystem"
They aren't talking about getting rid of them, so arguing for their validity is pointless. You fell for their strawman argument!
The issue at hand is: Is flagging a comment or post exactly the opposite of upvoting it, and should they be considered opposite sides of the same coin by the site and community?
Arguing that flags/downvotes are healthy is pointless. Nobody is saying otherwise. Literally nobody.