Zoro never challenged them to a 1 v 1. When Zoro makes his declaration and starts talking specifically thats when he does it. For example, he challenged Kuma and got pissed when people interfered. Zoro will jump you, but when he makes that challenge he don't want people to ne jumping in.
I remember that as he got pissed because he was trying to self sacrifice against Kuma like Luffy did with Aokiji, and it was hopeless to bring more bodies to the chopping block, not because it was interrupting a 1vs1 he thought he could win, I might be wrong, it has been a long time since I read that fight.
King didn't accept a fair duel and rightly admitted as such, Zoro too said he would engage in unsportsmanlike conduct to win at whatever costs (so much for low diff).
Kings method to kill Zoro by BFR is completely valid in a pirate fight, Luffy didn't even complain when he was drugged by a third party against Kat, wheras Zoro is just mad he can't handle King's fighting style and wants him to fight more orthodox.