It says right there in the option that the chance that Odyssial will even live long enough to find Larsyifex again is diminishingly small because Creation is just that big and with the local civilisation destroyed Odyssial has no idea where Larsyifex will go once he's finished here with whatever he's doing here in the first place.
Odyssial doesn't know Exaltations will even be a thing.
All he has available is what is available for mortals and even getting his hands on Sorcery seems like it'd be rather unlikely.
If he leaves now, as far as he knows he'll first have to actually find a way to make killing Larsyifex as a mortal viable at all and then he'll have to track Larsyifex down, even though Creation is big enough that it is literally impossible to travel through even a fraction of it, and he'll have to manage both in a human lifetime.
All Larsyifex has to do is go somewhere far away on a whim and, barring Exaltations which Odyssial isn't aware of being a thing, it becomes literally impossible to even get another chance at killing him.