He should know that these two just lying around, he should make a plan b and c in the case if they would break up the promise. Just call out Whitebeard or Rogerpirates and he would take them out of Wano, but I think it also was is proud that he didn´t call them for help. But in wano case he should do that, with this his glorious life end in pain and bring wano in choas...
Only that would have made for a crap story (or in other words, none). And it's a Wano-issue, and Oden deems himself as a quite capable fighter one on one, so one can't really expect him to do some gimmicky Super Smash Bros crap (that's cool when they are all from elsewhere anyway). And he actually was on his own, among his own people.
Oden is sane to go step by step, one doesn't have to account for any insane, evil person's hypothetical twists in advance, that's how they ultimately show themselves for what they are, and it's just degenerate and "degenerative", serving just lesser or no ends. After all, anyone's brain might hypothetically malfunction or they are dumber than one realistically expects, which however isn't their better quality.
And of course Oden tried to kill Orochi and Kaido first, but his first attack was unsuccessful (when he had just returned), so he just did the best he could do in the moment, not being a total maniac either who might have run himself out fighting all alone that moment, and he is someone who can talk and listen besides.
I guess he was counting on the representative value of the true leader or figurehead of Wano dancing like a fool, repeatedly in exchange for the lives of his people, being basically enough to appease Orochi's revenge. He might have expected
those hundred(s) of people he saved (did he or didn't he? Apparently he did so far) were the extent of Orochi's revenge, rather than expecting him to want to literally kill all the country, until forever. And Kaido also had a word in this, as the true force and a pirate, and he might have expected to rely more on a pact with a pirate or that simply his scheduling is his priority.
He later did a full-on attack (which was always an option), which was quite close to successful, but luck wasn't on their side. And that's how he got into this situation. Kaido apparently did not go back on his word to not boil the others alive, and I frankly find such a basic promise within his capacity. However it even goes without saying that he then won't leave them alone and have peaceful lives.
All the steps of the story were exhausted, fighting and doing a deal or trying to lessen the burden, and
Oden needs to show his character.
That's what's important, and always saying he could have done this and abcd, and preening oneself and sneering, is always pretty pathetic.
It's important to keep in mind, when those low rats in the chapter call him "Idiot Lord", it's just dumb gossip based on looks and because they're sadists (possibly not unlike in Orochi's backstory...), not because they know or care about anything (though simple looks etc. are probably also influential here...).
P.S.: It's funny how easily people are manipulated to now stand behind Orochi now, although it's quite predictable that such a part of a story always comes. Why don't you all marry Orochi-swan now...