It doesn't really have anything to do with the balance of power. If anyone else had Kaido's Devil Fruit, the strength of the Boro Breath would be the same. That's an innate ability of the Devil Fruit itself. Just because Kaido has that Devil Fruit, doesn't mean the attack itself becomes stronger. Akainu doesn't make his magma hotter, Kizaru doesn't make Light faster, Aokiji doesn't make his Ice colder, etc... Ace's Fire stalemated Aokiji's Ice, despite Aokiji being much more powerful than Ace was.
We've seen Kinemon slice up a mini nuke sized explosion before. So we already know just how capable his fire cutting ability is. It's not like he stopped a Thunder Bagua from Kaido. He stopped an attack which his character by design was created to be capable of stopping.