Yep that's incorrect, at least as far the missiles and claw based attacks go. Converted damage from kicks (Dragon flight and Dragon tail) are not effected by pierce at all, as they are calculated from the boot rather than the weapon and also calculated based on the physical damage done, before dealing the elemental damage. This stops other mods like crushing blow on the offhand from stacking with the main hand for kicks. It's also there to prevent a crushing blow kick doing: "use CB damage then multiply it by 3000% and deal as additional fire splash damage" to pretty much 1 shot any boss with a single crushing blow based dragon tail/flight.
So I wasn't exactly thorough with my reply. To put it better:
1: Any melee attack that uses claws is calculated with the pierce from both hands.
2: Any melee attack that releases a missile whos damage is independent of physical damage done to the target (Any charge up skill that releases a missile) is calculated with the pierce from both hands.
3: Any melee attack that calculates a base damage based on physical damage dealt, then takes that damage and uses it to calculate a secondary elemental damage type is not effected by passive pierce at all.
#3 Surprised me as I had always been under the impression that main hand pierce did work with kicks that use physical > elemental conversion, but after through testing via phys immune fallens, 100% fire pierce weapons and a poor zombie to initiate the kicks on, I can confirm, that is the case. After testing I did a little digging and found more detailed explanation of the routine and the possibilities of the CB issue I described above being the reason. Only directly lowering the targets resistance to the element (LR/Convic) works for those types of attacks.
I took screenshots of the tests, before/after if there are still doubts. I didn't sort them and post them atm because there are like 60-70 I need to go through to weed out the ones when I didn't highlight the mob correctly after a kick/attack to show the hp, or used the wrong skill, or missed the attack and hit print screen anyway
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Hope that helps!