QuadratureSurfer

Reading into it a bit more, they updated their algorithm in 2021:

In 2021, the IEA adopted for the first time a new hybrid modelling approach relying on the strengths of both models. The integrated framework of the IEA’s Global Energy and Climate Model (GEC Model) is now the principal tool used to generate detailed sector-by-sector and region-by-region long-term scenarios across IEA publications, including the World Energy Outlook series and Energy Technology Perspectives series.

https://www.iea.org/reports/world-energy-outlook-2025#methodology