Akainu making donut out of ace?
Same way hakiless rubber could damage Enel, because of elemental weakness. This was outright stated by Rayleigh.
Unless Katakuri is weak to flames (and I doubt it, because he has flaming attacks in his moveset), and even then Greenbull who should be weak to flames, simply reformed without damage when he was burned.
Bonus: Jackpot is not doing shit to the Invincible Katakuri either.
Perks of being logia-like.