There is no "German" school system. There are (at least) 16 different school systems in Germany. The original article glosses over that. Each state has (almost) full autonomy regarding schools, there are only voluntary (by the council of state education ministers) alignments between states. The federal government can only apply minimal directions by providing some additional funding in limited areas. And even within states there are often different kinds of public schools depending on the current political trends in that state at the time, e.g. to try out or introduce "Gesamtschule" (1-branch secondary school system) vs. 2- or 3-branch (and maybe fuse some or all secondary 3-branch "Hauptschule" and "Realschule" into "Mittelschule"), 8- vs. 9-year Gymnasium etc. It is a total mess, what might be true for one kind of school in one state might be different a few kilometers away in the next state.
So comparisons are hard, even between German states.
I also think this autonomy is one of the reasons why the overall average quality is so bad: Schools are a political issue that is used to distinguish a new state government from a previous one, and to distinguish one state from another. So with each election, you get state politicians running around like headless chickens, trying to "make things better" in schools by randomly doing "something", because what the previous government in the same state did obviously cannot be right. And what that other state governed by a different party does cannot be right either. ad inf...