I think the way relational trauma is understood today, at least as applied to children, is that the event breaks the person's internal sense of self and of safety.
Children are SO vulnerable, and their brains understand this on a lizard level. "Those people are huge and I am very small" is obvious, even to an infant. They know, instinctively, that their parents are what stand between them and death.
When a parent abuses a child, violates a child, the child doesn't doubt the parent's goodness or rightness. They can't afford to. It's much too dangerous. If the person standing between you and death isn't safe, nothing and nowhere is safe.
So they doubt themselves, their own goodness, their own rightness. The operating system gets installed with major bugs. It can't trust its own programs, its own judgment, whether it's even real and has a right to keep existing. It's broken on such a deep level that it can't recover on its own.
I think sometimes abuse doesn't lead to trauma if there are strong enough mitigating factors. She's now deceased (death totally unrelated to this) so I can't ask her, but I don't think my sister experienced our abuse as trauma like I did, because she internalized very early on that dad was unreliable and unsafe, so it didn't shake her sense of self and her own judgment. I have theories about why she was able to do that and I wasn't, but there's only so much to be gained by speculating in her absence.
I made my parent comment intentionally vague because I didn't want to cause debate about it, but the context where I normally hear people saying, "My parents did it to me and I turned out fine," is talking about spanking their children. So my response is, essentially, "Are you fine? You're standing here defending the idea of hitting a child. To me that's a very strong indicator of not being fine." I thought it was a common enough conversation for people to know what I meant without spelling it out and opening that can of worms, but that's probably culturally contextual.
Hope this at least clears things up a bit.