Nice. And since this is D&D, instead of the 50,000 survivors running away to live another day, your jerk DM would have them mob you and you'd die. And come back to life later.
Epic is seriously another matter entirely when it comes to whether things make sense or not. You made a really good point about HPs not being actual damage, why does that not apply in all cases?
So, when this epic power that you reference is unleashed, instead of burning the flesh off of 950,000 minions, perhaps it just does that to . . .100,000 of them, and the others are all so freaking terrified that they bolt like rabbits. Leaving 50,000 to charge you and make you die (of course, if you can repeat the attack they are reduced to 2,500, and you will not likely survive that either. The next round though, when there are only 125 left, you have a chance).
The recent article up on the WotC site about high level challenges addresses this issue, kind of. Minions are not a club to be waved around without skill. You apply them delicately, and deliberately. A minion rush will always die, even at 1st level.
Jay