"In running the line that undersea cables were cut, Greenwald is straying far enough from what's feasible and credible that his judgement on other claims needs to be questioned."
You might want to go back and read the original article (linked back from this piece (via the last one)). I have, and I don't see anywhere Greenwald saying that the cables were cut. The closest he gets is this:
"Top secret documents provided by the whistleblower demonstrate that the GCSB, with ongoing NSA cooperation, implemented Phase I of the mass surveillance program code-named “Speargun” at some point in 2012 or early 2013. “Speargun” involved the covert installation of “cable access” equipment, which appears to refer to surveillance of the country’s main undersea cable link, the Southern Cross cable. [...]
Upon completion of the first stage, Speargun moved to Phase II, under which “metadata probes” were to be inserted into those cables."
Then he goes on to provide his evidence for those statements.
https://firstlook.org/theintercept/2014/09/15/new-zealand-gcsb-speargun-mass-surveillance/
So you have "access equipment", and "probes", which could mean almost anything. Nowhere does it say "cut".