He did not arrest her he attacked her.
Actually he did arrest her. He also handcuffed her and placed her in a police car. When the police restrict your ability to voluntarily leave, they have seized. Being “seized” by the police while not technically arrested is tantamount to arrest. As is being detained. The police are reluctant to say, “You are under arrest for _______[fill in blank] because when they say,”You are under arrest” then your Fourth, Fifth, and Sixth Amendment Constitutional Rights kick in. That is when the investigation switches from investigatory to accusatory ones Constitutional Rights kick in. Abe Fortas wrote an informative essay on that. And he did inform her that she was under arrest.