The official report the original BBC article is based on is available here: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6.... Definitions are inside, but the big ones he mentions are Intergovernmental Panel on Climate Change (IPCC), AR6 (Annual Report 6), and WGI (Working Group I).

Essentially what he's saying is that the conclusions in the report are based heavily on the more-extreme, though less-likely outcomes (i.e., exaggerating the truth of the impact of climate change based on observational data by using extreme models/predictions). The report also ignores his (Roger's) own heavily-cited research in favor of a less-cited resource that confirms their biases.

Edit: this is problematic because it generates headlines like the one this article is using and sends people into a panic spiral (as opposed to focusing on long-term solutions and actual science).