I just noticed another problem with this theory and it kinda debunks the theory entirely. Darth Nihilus was one of Kreia's students/partners in the triumverant until Sion and Nihilus decided to overthrow her.
Furthermore, Kreia states that, "He if he can truely be called a man any longer..." This implies she knows whom Nihilus was before he became Nihilus.
So I don't think Nihilus is part of the exile.
My theory is he is one of the Jedi at Malachor V, when the Mass Shadow Generator activated, Nihilus was practically at ground zero and he survived by taking in the life energy of everyone else that died, at a terrible price becoming like a vampire, needing to literally feed off the life force of others to survive.