Afaikt the paper refernced in the article is The Human Cost of Anthropogenic Global Warming: Semi-Quantitative Prediction and the 1,000-Tonne Rule. .

From the abstract:

The carbon budget for 2°C AGW (roughly 10^12 tonnes carbon) will indirectly cause roughly 10^9 future premature deaths (10% of projected maximum global population), spread over one to two centuries.

The key part of this being that it specifies 2C of warming. According to the climate action tracker, with current policies are on track for 2.7 degrees of warming by 2100. So assuming no further polices to move back to 2C or less, the 1 billion deaths could be larger.