That's where some fans can land in heated debate. There is a line Morbius utters (and I'm sorry I can't remember the exact words), something to the effect, "Just how far back can you go, Doctor?" As he's aying this, we see upon a monitor an image of Baker, then Pertwee, then Troughton, finally Hartnell. But it doesn't stop there. We continue to see faces, the production personnel listed above, each dressed anachronistically.
yeah, like you, I "write it off" as being previous incarnations of Morbius, but when that serial was being recorded, the "intention" by the creative staff (as has been stated in a few interviews) was that those faces were meant to be the Doctor.
Remember, they played fast a loose with the background material during the Hinchcliffe era. Until "The Deadly Assassin", it was assumed that TimeLords (and thus, the Doctor) could regenerate indefinitely, effectively being immortal. But to give the Master adequate motivtaion, the concept of a 12 regeneration limit was introduced. That idea stuck; the "prior to Hartnell" incarnations didn't.
Sincerely,
Bill