Well, that they said she is dead means nothing at all.
And her wounds were not so fatal, if you compare it to all the other stabbing, piercing and slashing that we already saw in the manga.
We can assume that she is still alive and will survive.
Why would Kubo wait 300 chapters and suddenly start killing of a major character. That doesn't really make sense.
Ulquiorra probably just said it to make Ichigo angry.
If you think about it, to let her appear near death and later show her dead would've been the better solution if Kubo wanted to kill her. Would be a much bigger surprise.
And let's not forget that Rukia is one of the most popular characters in Bleach next to Ichigo.
That might be important to Kubo. You never know.
"For Feanor was made the mightiest in all parts of body and mind, in valour, in endurance, in beauty, in understanding, in skill, in strength and subtlety alike, of all the Children of Illuvatar, and a bright flame was in him"