I read the same thing years ago. People were warning that using the same grocery bag over and over was unsanitary.
Those bags frequently have blood from thawing meats, rotten fruit and vegetable remnants, spilled yogurt or milk or whatever, in the bottom. Reusable bags can work as long as the users are willing to clean and sanitize them after each use. Most people just do not do that.
Unsanitary reusable bags is an old and well known issue.