That seems completely wrong my friend. As i said before when Arthas and Illidan met in Icecrown Illidan said "Hello, Arthas." and Arthas repiled "You look different, Illidan. I guess the Skull of Gul'dan didn't agree with you." that's where they interacted. that was after Illidan said
"You're out of your league, old king. You should have stayed hidden underground".
Anub'arak is an old king who has been living underground in his kingdom. I know for a fact and 100% sure that he was talking to Arthas i saw a thread about this out of the Hive before. The thread isn't about who's Illidan talking to. It's about whether Illidan know Anub'arak or no. Maybe no then. Maybe he just heard of him from his scouts just before the battle when Arthas arrived to Northrend and he and Anub'arak met Kael. Thanks for sharing your thoughts my friend