How does sunder make terror zones more difficult for Hammerdins?

No it doesn’t

Because

1st: Hdin still faces 0 magic resistance monsters while the charms only make other damage type at 95 which is 20x more damage mitigation that magic faces

2nd: to be closer to Hdin in terms of power, these builds have to sacrifice a gc slot+75 resist

So not only it doesn’t make it on equal footing, but it has a drawback which Hdin doesn’t have in order that have that power