I think the point is that the quantity of CO2e dumped into the atmosphere when you make a cotton bag has a much worse environmental impact than the plastic bag dumped in landfill.
[1] https://medium.com/@tabitha.whiting/should-you-swap-plastic-...
"The study estimated that a cotton tote bag’s total carbon footprint was 598.6lb of CO2e. This compared to 3.48lb of CO2e for a standard plastic bag. That means that you would need to use the tote bag 172 times for every 1 time you used the plastic bag."
[1] https://medium.com/@tabitha.whiting/should-you-swap-plastic-... "The study estimated that a cotton tote bag’s total carbon footprint was 598.6lb of CO2e. This compared to 3.48lb of CO2e for a standard plastic bag. That means that you would need to use the tote bag 172 times for every 1 time you used the plastic bag."