I'm the child of an abusive father too, and wanted to add some thoughts - my mother unfortunately didn't leave him until I was in my late teens, so please know you've already done the best thing you could do for her, which is to leave.
With very young children, as has been said, there's an instinctive drive to bond/attach to parents, even awful ones, because we cannot survive alone. She will be bonded to him at this point. He's her dad and she's 6. It's normal.
However I wouldn't necessarily attach the label 'love' to it. At 6, you don't really understand what that means. It's a label attached to the emotions you feel for close relatives. She has a natural, instinctive emotional response to him, and she has been told this is called love. You are calling it love. But it may not be.
By the time I was 8/9, I knew I didn't like him, and I think the realisation that I didn't love him either was around this time. It wasn't something I really told anyone about, no-one wants to hear children say that and you can be shamed and told off for it. By the time I hit early teens the plan was to get to uni and cut all contact.
But during this, he still expected me to tell him I loved him, and when I was pressured to say it, I did. It was the safest option. I learned early on that I could say the words but not feel them. I hated saying it, though, it made me feel sick. But I had to. I wanted to spit in his face and tell him my biggest hope was that he would die. Instead I told him I loved him because that's what he wanted to hear and I was too frightened to say no.
Please don't judge your daughter for saying it. She's a baby trying to navigate a relationship with an abusive adult man.