Did they specify the faces were supposed to be past incarnations?
No, they didn't.
Yes, they did. If you watch the story with no preconcieved notions it is obvious that the faces were meant to be pre-Hartnell Doctors. And no, it doesn't fit neatly with previous or later continuity, but it is what it is.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy