Hard to say as he may just have misjudged the situation in the moment and he has apologised, but I think you're right to be concerned.
My grandfather used to tease me when I was little and my gran would get really cross with him but he didn't stop doing it. I think he thought that's how you engage a child, but it taught me about powerlessness and how you can be mocked for legitimate self-expression. The way your DD didn't react until she was in her room makes me wonder if she felt powerless and like her opinion didn't matter so she waited until she was alone to express herself. How women see consent, who they see as allies, when and how they choose to stand up for themselves, they're all tied into how we feel about being listened to and believed.
Does DD usually feel close to DP, trust him to support and care for her etc? I was also bullied at school, parents divorced, crap dad etc, so this was not in isolation, whereas an otherwise stable and supportive home life would have made it less of a 'defining moment' for me (as you so astutely refer to).
So I think YANBU but not sure where you can go from here. Can you have a family conversation about respect, playing and listening, reinforcing that no means no? I don't mean this as a punitive thing for your DP, more a discussion and agreement, so you can all feel you're on the same page? And in the process you may feel more listened to yourself, and less angry?
It feels like DD is so young to be having this conversation, but equally we learn this stuff when we're young. Idk if anything I've said is helpful, but I do feel for you and I think it's great you've felt this deeply and you're taking it seriously. 