I've listened to the isolated tracks, and the harmonies are definitely separate performances. Vocorders don't just do robot voices, they are also used for modulation effects and there are a bunch of those on those songs. There is also something following the bass line on Billie Jean that could be a vocorder.
I don't hear anything in the vocal harmonies that MJ couldn't easily hit and we can tell from the unison parts that he can certainly sing really tightly with himself. There is no need for synthetic harmony tracks.