The FTL attack and >50% mining are not by themselves enough to keep difficulty low for more than a few blocks at a time before it has to rise back up. It requires something else to make difficulty go really low, like this powLimit attack. I have not analyzed the Verge attack, but I saw it had alternating high and low timestamps. This indicates the powLimit attack may have been at the core. The powLimit attack almost always requires selfish mining, but there are coin settings that make a public attack possible. So I think maybe the combination of N=12 and FTL= 7200 was what made a public version of the powLimit attack possible on Verge. The public version of the attack appears plugged by tightening the limits around it, but it appears the selfish mining version is still possible.