If he was attacked, which seems to be proven and accepted, then what’s the long sentence for?
Just because he was attacked, does not justify the use of excessive force. Self-defense is only a defense if the individual defending himself uses “reasonable” force; kicking and stomping on an individual after that individual is already on the ground is not “reasonable” force.
Mette did not lose because he punched the guy once. He lost because the evidence suggested that he did much more than just punch the guy once. Had the judge believed his story (i.e., had the evidence backed him up), he wouldn’t be off to jail.