Jesus told the theif on the cross he would be with him in paradise that evening? How is that any different? I looked it up. It appears they believe Mary was given her heavenly body already. Do they understand that no one has been resurrected yet? Not even Mary, Who is in Paradise awaiting the return of her savior?
Well, it's all rather confusing in my mind and I don't want to pretend to be an expert on such things. Also, there's a lot of differing opinions even among those who are in the same Faith.
Anyway, some see Mary as Divine.... she had to be, they reason because something un-pure couldn't conceive Jesus, the Holy One. (cf. The Immaculate Conception and The Council of Ephesus).
Therefore, she also could not die.