I would say that "being defensive" is different to "defending oneself". Given the semantic overlap it's very easy to see why they'd be conflated. Being defensive is an emotional response that serves to protect ones ego. It often lashes out, invokes absolutes, dismisses the other, deflects all responsibility, and avoids resolution. Defending your behaviour in an assertive manner need not do this.

Let's take an example. Say that my wife finds the cutlery draw in a messy state. She's previously brought it up with me and I had agreed to make an effort to help keep it tidy. It's potentially ambiguous in that it's not clear who's "fault" it is. But that doesn't actually matter in terms of resolving the conflict.

Defensive me: "Huh? I dunno! I've just been putting things there like normal as far as I can remember. And anyway I've been having to do the clearing up as well as putting the kids to bed this week so what do you expect?! You're always taking me to task for stuff like this. Why are you so wound up about a draw? If it's so important to you why don't you just tidy it yourself?"

Assertive & empathetic me: "You know what, you're right. The draw is in a state. And I can understand why that's upsettting as you have brought it up before, and it is frustrating to have to root around to get what you need. To be honest, I don't remember being very scrupulous about keeping it orderly, but I'll make sure to focus on it. I've been finding it a struggle to stay mindful about what I'm doing in the evenings this week as I realise I've taken on quite a lot of chores, so it's very possible I overlooked this. Shall we try doing XYZ to make it easier to place things back neatly?"