In che modo è legale che i datori di lavoro tedeschi richiedano solo oratori tedeschi “nativi”?

Native German speaker does not necessarily mean that the applicant(s) may only be German, but that the language must be native German. I’ll read through the ad again, I used to work in HR and see if everything is correct. But my statement is the normal approach and there are many who can speak German at native speaker level without having German citizenship.

I think what they mean is native level. Employers should specify the required level of proficiency in the categories (e.g., C1 or C2 according to the CEFR).