all possible strategies for playing as Inferno against the Elves.

so what's the point of the dark replenishment?

So, he'll control the dragons (and why not, if his dark magic is skillful), but it won't work. Then it's better if the hero doesn't get pushed back too far and can move again soon.
But maybe you're right. Although, mathematically, slowing down seems more useful than an acceleration of the same strength. But if it doesn't work on the strongest enemies, that will be unpleasant.