In our experience the paper, plastic and cloth bags you purchase (we have two cloth ones from years ago and they are huge in size) are located under the conveyor belt (or thereabouts) so I could see how someone might think they have none to purchase. I actually bought some very inexpensive reusable bags from Aldi that fold up very small into themselves. Those are good for lighter items especially if you're just popping in for a few things and don't want to bother with a cart.
It also may not be common knowledge to some if they don't shop there enough to know that you can just grab the cardboard boxes to use. The deeper boxes are good for pasta, cereral, etc. Shallow for other things.
Aldi does use a different model than others but IME they haven't "gone away" with single use plastic bags, rather their business model was already just different. The cashier doesn't bag your items, your cart doesn't usually stay your cart, you go off to the side to pack up your items, etc.