I think most of the taps they're referring to involve tapping light directly off the fiber, which reduces the light intensity at the receiving end and can therefore be detected in theory at least

Interfering with existing amplifiers/signal boosters would be fairly hard to hide unless the NSA was solely responsible for that units maintenance, and I suspect that as a general rule they aren't. It's much easier to hide a tap at some random point along the line where nobody has any reason to visit