and I believe they are also unionized? Or just have more labour protections.

Well first off there are two Aldis in Germany, Aldi Süd (south, I think this is the one active in America and Austria) and Aldi Nord (north). Since they are different companies, they will have differences in how they treat labour organisations.

Genrally speaking they will both of course oppose them though, as companies are wont to do. For example two month ago Aldi Nord fired a member of the works council (something like a union rep, but independently elected by all employees), which is a rather... uncommon occurrence due to their legal protections. Their lawyer is optimistic.

The companies are unionised to a degree, for example the aforementioned work council had 4/19 members coming from Ver.di, a cross-occupational union for service workers of all kinds. It's not uncommon for big employers like the Aldis to have several unions active among their staff, but all employees being union members is also rare.

Overall though it's probably mostly the labour protections by law that you are seeing at work here. For example the possibility to create these work councils, which can have quite a lot of influence on company policy, is mandated by law. You have to announce a company wide meeting and then the quorum to elect a representative at that meeting is three(!) people, no matter the company size.