The books forward mentions many translations from German into other languages so I'm wondering if my confusion is due to it not being called the artillery terms I'm used to hearing. They are in a salient so I know the flanks have something to do with the angle of the gun trajectory?
edit: word
Junger's description of the German line forming a salient indicates that the German line is projecting or bulging into the British line. This means that the German salient is surrounded on three sides by British forces. Assuming Junger is located at the front of the salient, or the portion of the German salient that runs roughly parallel to the main German line, his position would have been exposed to artillery fire from both sides. The danger of this is that due to the direction that the German front line ran, artillery stationed along the sides of the salient but in line with the front trench would possibly be able to fire along the length of the German positions. The crude diagram below displays this: https://imgur.com/a/0gGV7
I haven't read Junger so I can't say for certain that is what he is referring to but from your description, it is likely as I have described.
In the German text Jünger writes "Flankierungsbatterie" which was obviously directly translate to "flanking battery." From context I would understand it as an enfilading battery, however googling the word it only turns up Storms of Steel and in the description of two castles.
/u/TheWellSpokenMan gives a good answer. This seems to be what Junger means when referring to flanking artillery.
The only other scenario I can think of is the use of artillery to block movement to the flanks of an advancing force in an attempt to canalise them. But as you say it's referring to the force in a salient, so you can probably discount this one.