The description of the STEM/MINT situation in German universities is far too generalized. You have quite a few STEM/MINT universities and programs which require a numerus clausus, its just up to the university to set the NC. Many classical STEM/MINT programs dont have a problem with having a few hundred students each year instead of a few dozen since alot of those will fail in the first years where students just have to learn the basics of the field. Those courses are generally easy to teach and require little interaction with individual students. In the most prestigious technical universities having 70 to 90% of the students fail specific exams is nothing special. And studends are barred from the program after failing the 3rd try. You are also not completely barred with to low of an average entry grade from school, you will just have to wait longer to start your degree. That part is the case for every university and every program, STEM/MINT isnt a special case here.
edited for more information