They didn't enter the barcodes from memory, they entered the price of the item from memory. That alone is a feat, though.

Back then they had much less variation in inventory and very limited price groups: all products fit into the ~10 tiers they had, ranging from 0.19 to 3.99 or so.

The increased variation in inventory and the scanners came at about the same time, and they only did that when they figured out how to keep their famous checkout speed. (and it's still slightly slower than it was back then)

Yep, during peak hours you feared Aldi checkout back then.

When the queue was full the best cashiers would really stress you by observing you putting stuff onto the belt and already starting to enter prices ahead of them reaching their area (where now the scanner is). Half way through loading the stuff into the cart again the cashier would urge you to pay and the next customer's items started bumping into yours. It was very efficient and stressy, best you were two guys to share loading/unloading and paying (back then payment was only possible in cash).

The scanners eased that a bit :)

Not really. I worked in a Winn-Dixie in high school during the pre-barcode era. The products were marked with a price gun that applied a sticker. But of course, sometimes the can of corn didn't get a sticker or the sticker fell off, so you learned what the price was to avoid holding up the line while you called for a price check.

Side-effect: I would have cleaned up on "The Price is Right" if they'd let a kid on the show, since I knew what everything cost.

Aldi Nord entered three-digit SKUs, Aldi Süd prices (both of which where strategically assigned for faster typing speed for common items).

Germany uses EAN-13, not UPC.