I really don't know what's the big deal. I mean sure it's amazing and all, but it's just a composite. And one of the simplest do to, in terms of technique. Please don't get me wrong, I know it wasn't easy... All the planning, shooting, post work and everything. I'm amazed, but theres nothing technically and workflowly (I bet I just made that word up) new or difficult about it.
Anyways... to answer your question. They could, but I personally think that would be a waste of time. Since it was a steady shoot I think they just used a simple mask - a general outline, or difference keying. Hard to say from such poor youtube quality, but that's how I'd try to do it as my first approach.
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