Damage Taken As - why?

So I've seen a few builds this league that use Tempered By War + Watcher's Eye + other sources of cold + lightning damage taken as fire. Or the gg rhex talisman amulet everyone on this sub is recommending. Then they add Max Fire Res to defend more.

The one I'm looking at right now is Carnarius' Str Stacking build. My question is that his Ele Max Hit is already 65k without it, meanwhile his Phys max hit is a measly 15k and Chaos max hit 32k (Immortal Call turned off).

Just by omitting this strategy and picking up some different layers of defense - Flesh and Stone instead of Purity of Fire, you can bring both of those up to somewhat respectable numbers. It also lets you swap the watcher's eye to something more offensive, so I ended up taking out some dps nodes for armour. Just with these changes, we're seeing a Phys/Chaos max hit of 20k/42k respectively, with barely a difference in DPS.

I added a Phys Defense loadout on his PoB so you can compare the two: https://pobb.in/-qSBmtq7kkjq

That being said, I'm currently leveling a char to run this PhysDefense setup (just hit 80 in t16s) and I was getting chunked here and there. Am I hallucinating here? Is this damage-taken-as setup better than the Max Hit shows? Why am I seeing so much talk about the Sublime Vision/alternatives?

TL/DR: Damage Conversion benefits don't translate well to PoB max hit. What gives?