It is also detectable: Long haul folks sometimes pull up the cable to service it for various reasons. So they pull it up and see a zillion fiber taps? WTF?
And where does that cable tap go?
To your bigger point: Yes, this article has zero explanation of how it is done. The salient sentences are:
However, light signals can not be read in another way. Therefore you have so far to expose and turn that a small part of the light waves emerging from the fiber, the individual glass fibers. If you immediately reinforce these waves again and transported via another cable, it has diverted the data.
To anyone in fiber comm, this language is pretty much gobbledegook. Any chance the pastebin is a translation? It vaguely sounds like someone is describing a fiber tap, but in such confused language as to make it sound like a bad spy novel.
Edit: I should have read through the thread. It is a translation: https://news.ycombinator.com/item?id=5982681