Arcane Nexus is just the ultimate gem generating global, it's
not a direct attack on anyone, unlike Utterdark or Astral Corruption. Rather, it's the same sort of claim on victory as Forge of the Ancients.
As such it's not considered the same as cancelling all NAPs, but rather a reason for the caster's neighbors to cancel NAPs and dogpile the said caster.
And of course, often it's done as a team effort, so who ever decides to attack the AN caster has to prepare for other nations to show up to defend the caster.