So I took my Demon Hunter to the Well of Eternity instance...
While there I tried to check a rumor and it's twoo! I'ts twoo!
Illidan does know the glaives. so here is the theory.
*Puts on tinfoil hat*
Illidan meets the future holder of his weapons. He then later takes them from the demon Azzinoth. Now he wouldnt willingly pass them on so he must of been killed for them. This prepares him to not be so arrogant at the Black Temple and lets him escape with a 'faked' death.
Speculation is fun :)