It's not outright stated, but it's implied. Kaido said there was a traitor in Oden's group, and came to face Oden before he even attempted to assault him because he was already prepared for it, said that Oden was too naïve, that he needed to reduce Oden's forces while increasing his own, with the narrative saying Oden was even stronger than Kaido expected just before getting sliced in half and in a position of getting defeated until the hostage moment started