That matters why? The instant we have characters that throw fireballs throws out all need or necessity for 100% accurate bible or other reference information. Arguing what "Nephilim" is in a fantasy video game by quoting the bible does not work. The bible doesn't include fireballs, big tentacle bad guys that shoot a ring of red skulls at you, worldstones, zombies and other walking dead creatures you find in the game.Here I leave you something from Wikipedia: Nephilim are beings who appear in the Hebrew Bible
It's like basing the logic of cars on the bible. Automobiles have nothing to do with the bible thus you would never base it off of it. Nephilim in this game has nothing to do with the bible's Nephilim.
Reading the text, it does not say "we are the spirits of the nephilim." It says "We are the spirits of Nephilim." This changes the semantics of the sentence drastically and Nephilim is no longer in context of a race and is now in the context of, logically, a place.
And what are you talking about Tyrael gave no explanation as to why you need to stop Baal from controlling the worldstone. You're stopping an evil demonlord from controlling a near infinite source of power that holds the planet together, how is that not enough of a reason? Case in point, Baal is the lord of DESTRUCTION. He wants to destroy, controlling a worldstone, he would do such a thing.
Here's my suggestion, Record or type down everything every NPC says in game. Put it all together in the chronological order in which it's said, and I guarantee you their will be a functioning story. And when I say every NPC, I also mean the old crazy dude from the cinematics.