The actual linked article isn't the paper, and doesn't even name the paper as far I can tell, it's just given as "Hu, W. et al. Science 381, 979–984 (2023)." in a footnote.
This is the actual paper, which (presumably, I cannot access) is specifically about how they made the estimate: Genomic inference of a severe human bottleneck during the Early to Middle Pleistocene transition:
This is the actual paper, which (presumably, I cannot access) is specifically about how they made the estimate: Genomic inference of a severe human bottleneck during the Early to Middle Pleistocene transition:
https://www.science.org/doi/10.1126/science.abq7487