It's a square shot, because Callisto follows and stays close in her space when she's stopped by the wall. On a mortal opponent, she'd have just bought herself an opening to get out if not completely away.

On a god - or, honestly, Callisto in general; she'd have at least bled before, but she's taken enough beatings in her life not to let it stop her - it's more like smacking a horse on the nose and Callisto makes a sharp sound of surprise and then backhands her in return for her trouble, hard.