It's better to reuse a bag, not have disposable's full stop. I know what he's getting at though. The disposable bags still take energy to create and then either rot in landfill or get recycled. Paper creates methane when it biodegrades. Plastic just stays there and breaks down into microplastics, but it doesn't release greenhouse gasses. There are arguments against both, and a strong argument in favour of reusables, but I appreciate that's not always suitable when you're in a grocery store wanting a portion of something in a clean bag.