You misread their statement. They said "8 degrees". With 8 C increase in temperature, what they described could very much happen. We expect 4 C worst case, which is why we do not need to worry about extinction. I think they are saying that we do not need to worry about more than 4 C increase exactly because people in the past fought for the cause, and if they stopped fighting the same way today some people feel resignation and want to stop fighting, even 4 C would have been too optimistic. The fact that people in the past did not resign themselves to the status quo is why we do not need to worry about an 8 C increase.