He is basically saying this is plausible. And his is the most sensible answer here by far IMO.
The whole point of "bad case" scenarios is that they involve feedback effects and breakdown of "civilization" as we know it.
This article as a whole is an illustration of the narrow, conventional thinking.
NB: i came here via https://passivehouseaccelerator.com/articles/building-our-solarpunk-future where they are citing this as evidence future won't be too bad:
For many of us, it’s all too easy to imagine the terrible, particularly as we witness the damage caused by just 1.2°C of global heating today. We’re also bombarded by Doomist messages.
For example, Roger Hallam, the co-founder of Extinction Rebellion, recently said this of climate change: “I am talking about the slaughter, death, and starvation of 6 billion people this century. That’s what the science predicts.”
Only that’s not what the science predicts. According to the fact-checker website, Climate Feedback: “Research shows that continuing climate change results in a broad array of serious threats to humans and other species. However, counter to Hallam’s statement, published studies have not predicted 6 billion human deaths this century and there is no credible mechanism referred to justify how this could happen.”