Why does he says that he'd been fired as Ultimate vs Supreme judge?
I don't think I've ever seen that man before or my memories failing me.
You're right, he's never appeared before. He's just a new character made up for this chapter just to rub off Yamaoka and Kurita's actual chemistry on Dan's, Mariko's, and Kinjo's faces.
Apart from that specific reason, we can just think of the whole thing as the judges being selected off-screen a long time ago before the start of the Ultimate vs. Supreme Menu battles. Which explains why this guy only showed up now.
Dan has been biased this whole time because of Kurita, but mostly to make Yamaoka look bad, so if anything, he's been biased in the other direction. You could argue that all of his decisions were consistent with the results of the matches, but he's done stuff like try to force a match in Australia just to stop them from getting alone time or give Yamaoka a hint on Yuzan's dish to make Yamaoka look like he didn't earn his win.
It's pretty obvious that the guy is no saint since his debut, but for me, it's ultimately confirmed during the Salmon Match-off arc.
While Yuzan's no-holds-barred criticisms about Yamaoka in that arc are particularly brutal, it's more than obvious that Dan also uses the same momentum to just eradicate Yamaoka once and for all from the competition to win Kurita's heart while putting up a righteous face, which just confirms my suspicions that he's just a scumbag, at least when it comes to the manga's love triangle/square/whatever.