That piece of the IPCC report that article is cherry picking is a.) a bit out of date now and b.) assuming a warming of 2C which we've already given up on, at least according to the more recent report which framed our current condition as a "code red"[0], though I find it very hard to believe that you missed that press release this some.
And to be clear, existential risk doesn't mean I'm claiming "we're going extinct!" it means that that lies in the realm of realistic possibility. The IPCC reports are well acknowledged to be fairly conservative and fail to incorporate a lot of unknowns (Thwaite's glacier is an obvious example of this).
One example of evidence for extinction risk is work of paleontologist Peter Ward. Ward's "Under a Green Sky" is his popular science version of some of his work (and worth a read). Ward discovered that quite a few major extinction events in the past have been connected with sudden rise in atmospheric CO2 and rapid climate change. He makes well reasoned arguments about how quickly conditions can change and how that could easily lead to massive extinction events (we're already in the 6th largest extinction event right now, which again, I'm sure you know).
The main issue with climate change is there are many positive (and of course some negative, but we don't quite now the balance) feed backs that we don't understand. It is quite possible that enough of these trigger and all of our "worst case" projects turn out to be quite optimistic.