Problem is, it wasn't the only major ley line in Suramar built before the War of the Ancients. This is shown in quests in Suramar by Falanaar, where another ancient ley line was used for another Arcan'dor tree, which of course exploded and created the Fal'dorei.
There could be some sort of connection between arcane and void magic in Suramar - the Nightwell's creation has been explained, but not why it is so much more corrupting than any other well of power. But, N'Zoth is most likely nowhere near Shal'Aran. The Broken Isles, perhaps, but not in the center of an abandoned town in Suramar's outskirts.