I just finished rereading that part because it bothered me too. He doesn't actually hit her, like with a closed fist or anything. It's this intense scene where they're arguing in the kitchen and he grabs her arm, hard enough to leave a bruise later. She pulls away and bumps into the counter. The book calls it an 'accident' stemming from his anger, but the line feels deliberately blurry.
What sticks with me is how Colleen Hoover writes his panic afterwards, his apologies. It's supposed to show his internal struggle, I guess, but it also primes you for the escalation later. So no, not a hit in that moment, but it's absolutely the first physical marker that his anger isn't just verbal. The story treats it as a serious violation, which it is, even if it's not a punch.
I remember putting the book down right there and thinking, 'Oh, so that's how it starts.'