I just read the article from the Keyboard magazine, it also said it was used as a master keyboard.
Hmm now I can only start guessing, but what about the Minimoog? (I will forget all the forbidden livestuff talk now) It was with him at Akropolis just above the K2000, it was behind the K2k during the Oxygene tour (I know it was said to be used for filtering drums), it was next to the K2k in the studio picture, it was "used" for the lead in Egypt. At last, I have never heard about what he really used it for on the Oxygene 7-13 album.
But as I said, I can only guess. I can't judge whether the lead sounds to be from a polyphonic synth. I could get a sound close to from the MinimoogV even using monophonic mode.