That doesn't sound right. I'm pretty sure MU Jadzia was Dax, given her relationship with Sisko. Didn't he still call her Old Man?
I thought Ezri killed Jadzia, but even if I'm wrong about that, Jadzia could've died some other way.
I think Sisko called her "Dax" at one point. She doesn't correct him, but maybe she just didn't feel like it. mu-Jadzia didn't have the same relationship with her Sisko as in the RU; I think their mirror selves were just ex-lovers. (I find it unlikely that the mirrors of Sisko and Curzon even met in the first place.) And from the look of it, they didn't break up on the friendliest of terms.
And mu-Ezri did not kill Jadzia. That much I know.