After doing some reading I am now of the opinion that the jury is out on this.
I found a bloke who wrote up a hypothesis to try and get funding for this exact type of experiment but, couldn't find if he went ahead or not. I had assumed that they are immune to their own venoms, but what I should have assumed is that, this is venom (and because of the nature of venom), there are many factors governing this.
I have found a paper by Hoser that seems to prove that death adders are immune, others believe that because the venom apparatus is in the mouth, and any venom expelled is swallowed (either in food, or just dripping into the mouth) and is therefore contained in an area that is not able to be affected (digestive tract), where if being injected into muscle or organ it will be affected.
There are also those who believe that some older snakes are more immune then juveniles, as they build up their immunity to their own venom over time (through digesting it).
There is some interesting stuff on venom doc forum re the bhp's, but to many big words for me. I prefer to deal in layman's terms!
IV it is possible that that snake copped a fang in an unlucky spot. Also possible that its own immunity was very low due to having a very good digestive system. Also possible that they are not immune and it was bitten and succumbed. Also possible that it died of some type of injury it received previous to the fight.
Ive only spent a couple of hours on this as it is pouring rain and I can't work today, so if anyone else has something more concrete, it might make for an interesting thread.