It is about the right shell, which is still visible after installation. Therefore, it is NOT at the edge and thus not allowed. Basically, the guideline states that the shell must not be visible after installation.
And as I said, as long as you haven’t paid, you are in a pretty good position and it would be a matter of honor to deliver defect-free goods. Once you have paid, you have to consider whether it is worth making a big fuss about it now. If they don’t respond after a proper notice of defects, you have to consider whether you want to push the matter through in court. Where is the window in the end? A basement window is probably less of an issue than the representative huge glass front in the living room.
PS: Who actually says that this guideline is legally valid at all? Building according to DIN, for example, is not a given, unless it has been contractually agreed upon. Any guideline is not necessarily binding, especially not when it is not a real guideline but just called that.