The item, if it's the one I am thinking of but can't remember the name of, doesn't give immunity to Energy Drain. It just absorbs negative energy attacks or death spells that the wearer would otherwise take, until it's depleted by 9 such attacks. Basically it's more like a False Life spell that gives "temporary levels" instead of HP. That's sufficiently different from immunity, not to mention expensive and limited, that I can understand Zaer's reasoning for it working.