I think that fight is the only time in the entire game where you'll see a Shadow packing more K/E DR than a Guardian or a Shadow. Shadows are just so friggin' disgustingly amazing at it because they get to have the best of absolutely friggin' *everything* mitigation wise (K/E DR, I/E DR, self-healing, shield/abs, def). I'm not sure how the Incinerate Armor debuff on Firebrand works (I'll check out my character info next time we do it though), but, if it's just a 100% armor debuff (rather than something like a 200% debuff so that, no matter how much +armor you're packing, you're still reduced to 0 armor rating, since you can't go negative) that's been coded in the same way as all of the other armor debuffs, I expect that the same exact thing would happen: Shadows end up with more armor than the other tanks by virtue of having it reduced by less, contributing another really rare/weird/abnormal situation in which they become the undisputed kings of all things mitigation.
The tooltips for the IA debuff are wrong actually. If you look at the script files, in SM/HM EC, the IA debuff reduces armor by 160% (not just 100%). I'm assuming this is to prevent shadows from just loling through the debuff. In nightmare mode, things are even worse: 160% armor debuff and
a 100% increase to damage received. Note that the damage received increase is from all sources, not just Firebrand, which is why you get hit so insanely hard by some of Stormcaller's attacks immediately after the tank swap.
With that said, if you take the three tanks and remove their armor entirely, shadows come out *vastly* ahead in terms of survivability. I've taken IA on EC HM many times without the tank swap, and even without Deflection up, I usually come out of it alive. I don't *like* to do it that way, but I can. I've heard of vanguards doing it as well (probably leaning on their incredibly high shield chance), but it would be mathematically much harder to pull off. Guardians have no hope whatsoever.