Vaal Fallen Elemental Hit appears to triple or quadruple dip on added chaos damage

Enemies effected:

Vaal Fallen, Colossal Vaal Fallen, possibly more including Vaal Slayer, all enemies that have the "Elemental Hit" ability

Expected Behavior:

These enemies utilize a pseudo "Elemental Hit" ability which has this wording:

"

Each attack with this skill will choose an element at random, and will only be able to deal damage of that element.

Based on the wording of "only be able to deal damage of that element" and the behavior of ele hit, monsters with this attack which roll "Extra Chaos Damage" should not gain any effective added damage, as their elemental hit attack won't allow added chaos damage.

Instead in this clip you can clearly see the enemy kill me with a chaos hit, and furthermore the damage on it is incredibly high to an unexpected degree.

https://streamable.com/fahgn7

In this clip my character has 67k armor (without any buffs up), and 30% chaos resist. The enemy appears to kill me in a single hit despite me having ~3.1k life.

Doing the math on expected vs actual outcome, it appears that the modifier "Gain 15% of non chaos damage as extra chaos damage" the enemy possesses is granting an enormous damage boost.

The conclusion seems to be that this effect is "triple dipping" off the elemental hit added damage types and completely ignoring the limitation of "and will only be able to deal damage of that element"