Their government likely never apologized, true. But we've seen that civilians (and even a few military ones) felt very differently. Some were victimized by that same government. After Kira's encounters with Marritza and Tekeny Ghemor, she should've been aware of that. Marritza even tried to give his very life as an apology.
She may have been aware of it, sure. But the particular Cardassian she was talking to HADN'T apologized. No, he had in fact just murdered several of her oldest friends. I think we can forgive a little hyperbole in the heat of anger.