uhm, haven't heard the thing about immortal npc's myself, so i can't say for sure how it was meant,
but just for a little something that might clear things up:
in fo- and tes games, there's not TWO states (normal killable npc / immortal "essential" npc), but there's a THIRD state, that's "protected", meaning this npc can't be killed by anyone BUT THE PLAYER.
most required npc's already were "protected" in skyrim, nv etc, and i'm pretty certain that's what it'll actually be, simply because totally unkillable wouldn't make any sense, immersionwise...