I do think it's important to identify specialties, and while these terms are maybe too universal, there's a major consideration that should be taken, given basic assumptions.
Yes, "the best defense is a good offense" can apply here and that's sort of what you're using. When they gain possession, can the d-man control the puck and either skate or pass it out of the zone effectively? That transition is a core trait of a good offensive defenseman. If talking about Ducks prospects, from what has been observed, we can use Z and Minty as examples of this.
However, how good are they when they don't have possession? How do they pressure and prevent chances? Are they able to use body size/strength to push opposing players out of the crease & off the puck, or to win board battles? Or do they have to rely on good stickwork, prediction, etc.? What I think we would hope for from a "defensive" defenseman (or 2-way) is that they are well-rounded here to just prevent opportunities overall, and that does include a physical element from a player that is relentless, has size, or can use physicality in other ways that someone small like Zell wouldn't be able to.
And yes, the Ducks just need better defensemen overall. But while it might be fun to have a player like EK, he is the epitome of an offensive defenseman. More goals and production is great, but I'm pretty sure at this point the team would benefit from someone that isn't a black hole in their own end.