If ALDI were the only supermarket which suppliers were able to trade with and customers could buy from then that would be a monopoly.

In a monopoly structure, one company tries to totally control the market, pushing out all competitors so that everyone has to deal with that one company and pay whatever that company wants to charge.

I don't think that Aldi or the supermarket industry is a monopoly. At least where I live, there are about five major supermarket companies with stores scattered about. I can do business with whichever one I want. I can also do business with online retailers, such as amazon, that sell most of the items I would want.