@KhoI mean, when you reach the deity level of power, there's no point in lying. Thus, an oath from such a being is that much more believable, since if they planned to break the oath, they needn't have made it in the first place. They could just bash your head in and be done with it.
You say that, but what happens where two gods of more or less equal power swear by Amul? It's not like they could exactly force anything out of the other without potentially procuring serious damage, and so lying may be better.
And it's always better to enter deals with creatures, even if they're weaker than you, for the sake of keeping up a good public image - otherwise, the other gods might get annoyed and decide to give you what's for. The equivalent of CKII's tyranny or EUIV's aggressive expansion >.> so lying there may be good for the sake of getting what you want by more cunning means while preserving a good image