Author here. It's the culture of the site. Like Reddit. Much more critical.
Also, remember that Medium is the platform where the writer writes. Commentators mean in fact to address the author. That helps for civility.
I'm ambivalent about Medium's approach. All in all, I do think that you get what you pay for. FB's negative impact is due to their ad monetization strategy. Medium is trying to charge through subscriptions vs advertising, and pays according to time reading vs. impressions, so is much more aligned with readers than non-subscription media.