That's a technique called splicing, specifically it refers to "splicing" two videos in the middle, in this case one with real input and one faked. It is surprisingly possible (and often easy) to detect splices based on minute details.

Splicing isn't always cheating, it can be used for other reasons.

I can't prove that he isn't, strictly speaking, but Kosmic is a highly respected runner and has the skills to make this record.