Their assumptions are worse than that. They assume constant global rate of renewable energy production but average global wind speeds aren't constant, they appear to be slowing down. This is called "global stilling". It is often blamed on climate change, which (it is now claimed) both speeds up and slows down the wind simultaneously, although the rapid construction of wind farms and urban buildings might also be related.

https://projects.research-and-innovation.ec.europa.eu/en/hor...