"He enjoyed reading all the fighting and dying stuff at the end too, which every character does in our game, because the player can do anything at any time, could choose to fight someone, could choose to betray them, could choose to do a favor for them."
If Esbern died as part of the story, this would never be included as it's such a big spoiler. It sounds much more like your character will be able to betray and kill Esbern if you wish, even though he is a key character. Also, he mentions that there are fighting and dying scripts for every character, and in light of the comment "the player can do anything at any time", all this strongly suggests that no-one is safe in Skyrim Except, of course, the children.
Do you want main quest NPCs to be killable without breaking the quest, or would you prefer they could be rendered unconscious, but never dead?
And just because I can, here's a transcript of Esbern's beautiful soliloquy:
I used to dream of it. In the dream I was standing some place high up, a tower or a mountain. It was always just before dawn and the whole world was in darkness. Then came the flash of light just on the horizon within the clouds that marked the border between worlds. It could have been lightning but there was no thunder. In the dream, the sense of foreboding grew, but I could never wake up. Then it came again, this time more distinct, closer, definitely not lightning, no, it was orange, brilliant orange – the color of hearth, and dawn.