Extra? You're telling me the in-game character the real main character was reborn into was so canonically strong that even the in-game protagonist and his companions combined couldn't beat him is... what, an extra? Be for real here. I see that the author is trying to push that whole 'loser was reborn into a meager, unimportant extra... but he becomes way stronger than the protagonist! surprise!' trope based on the title and labeling him as an "extra" but it lessens any reaction that could be had when the original in-game character was already stronger than the protagonist in every way.
Making the original in-game character die due to a sudden ambush from the in-game protagonist doesn't make him an extra. If anything, it instead is actually telling of his importance and strength since they had to rely on an unexpected ambush in his weakest state to beat him rather than confronting him head on. What would make him an extra would be if he was a weaker, less important character that had no significant impact on the story and that isn't him, based on what the story has told us about him so far.