Yup.I'm not even sure what you're seeing in the replay. No, he didn't aim at her. But he did clearly hit the ball too hard right at a person.It's his responsibility to make sure he doesn't do that. I'm sure there was probably a little extra punch on the ball because he was frustrated, but that doesn't even matter. If there aren't severe consequences for this sort of thing, then it will happen more often. Djokovic has just reminded players around the tour to pay attention.
The other thing is, if he had hit the ball in a soft, looping motion to, in effect "toss it to her", and she didn't notice, then it might have hit her, but I'm not sure they'd have defaulted him. A big part of it was that he hit it at her. It had some oomph. 90% of the time, they just catch it and it's fine. But if you wouldn't hit it at your 5-year-old that way, then don't hit it at a line judge that way. Leave no possibility of injury.