The Yonko overconfidence is perfectly in character.
They've been unmatched for decades, with all the Youngsters trying to make a name for themselves in the New World having to eventually give up and put themselves under one of them, or run back to Paradise.
They feel like they are invincible, and it's on purpose that the most inhumane of the Yonko who can't even be damaged by normal means are used to exemplify this notion.
They for sure still look down on the SN's, and certainly don't give their all at this point still, as just doing that would likely be perceived by them as a loss of sorts.