The clip comes from rushes, these are usually not stored, it's a different take, which after editing material to final form would usually be discarded as you rarely keep anything that didn't make it into broadcast for two years unless it's funny and can be used for DVD extras or something. So effectively, someone from production or editing team kept hand picked outtakes as leverage or press floggable item for those years and monetised the stash now.
There are two elements that could bury Jezza:
one - he was sure there was no material to prove Mirror claim that's why he denied it at first. Which means he thought all the rushes were gone, that they had control over those drives or tapes. He was certain they were discarded. Any experienced journalist knew that Mirror wouldn't run with the story unless they had a smoking gun. Effectively this treason must have been arranged by someone very close with top level access and grudge/agenda going back several years. Treason that close, from inside of the production team can destroy a man. You could even provoke buffoon behaviour just to get it on tape, if that was the agenda.
two - if there is anything else that was sold in a batch. Anything at all, any outtake, any other footage, in car camera, any wireless mic recording when they were off camera of any joke, jibe or private conversation that can be taken out of context and used against him, if timed properly, will take Clarkson off the air.