@Chrona
Word's definition aside, in most case it's just people's subjective view. One can saw it as confidence, but other might hate his attitude and called that arrogance.
Either way, being a bit arrogant isn't really a bad thing. Should Cosmo go into the match thinking "I leveled up, but I'm not sure if it's good enough"? Having defeatist attitude only made people giving up easily, so instead he could think "Yeah, I'm fcking strong, I'll win this!" and work his ass off toward that goal. You could say most fighters in this tournament are arrogant, since they always proclaim to be the strongest (until they got their butt kicked really hard).