I'd think in general, "just a broken back" would not be an immediate death sentence (even if it's results in you being paralyzed and unable to effectively save yourself). However I also don't see it being likely that's all you'd suffer from a grizzly attack. If they've broken your back, you probably have one or two other injuries.
If my spine is cracked I'm probably not making it out of the back country to get the medical care I need in order to not die.
I'd think in general, "just a broken back" would not be an immediate death sentence (even if it's results in you being paralyzed and unable to effectively save yourself). However I also don't see it being likely that's all you'd suffer from a grizzly attack. If they've broken your back, you probably have one or two other injuries.