Aldi is definitely geared toward low income. I am not low income but I shop there every week. One reason they pay more per employee is that they have many fewer employees per customer. Unlike most other chain markets, they will usually only have a single checkout lane and only open another when the line has more than 5 people in it, and almost never open more than 2 checkouts even when the wait is 10 minutes. The checkers are also the stockers, and everything else, so the few employees working at any one time never have any "slow" or "down" time. Customers have to pay for bags and bag all items themselves. They do not price tag each item (prices are only on the shelves) and often just open the wholesale box rather than shelf each item. They also save costs by offering few brands for any product, which means less shelf space and thus lower rent per type of product offered. Note that since most of the 20 brands of potato chips at a typical market are the same crap and mostly the same parent company, this reduced brand selection has little impact on quality or real variety.
