Well, the scientists are talking about 95%, which is 400 million people, and if people started dying out due to climate, you would see regions where people have a better time living. These would most likely be in the temperate bands, which are a narrow strip across South America and Africa, and a larger strip across North America and Eurasia. Those northern bands are thousands of kilometers long, and people have traveled those distances on foot before. Moreover, those 5000 people don't have to be in one place, they need to join up in a few generations at worst. Also, climate collapse isn't instant, as we are experiencing it right now, so those 5000 can start congregating before the collapse is complete. For reference, 0.1% of 8 billion is 1.6 million people. 5000 people is a third of a percent of that.

Killing every human is pretty hard.

Dude we made it through that already