That's the premise of the fight.
We know from Sanji's fight with Queen that the heat of his flame depends on the strength of his haki. As long as Sanji's haki can withstand it, he can keep raising the heat.
We saw that GB is fire resistant but not fire proof. Raizo's flame technique had 0 effect, while Momo's boro breath broke his defenses handily.
So Sanji will start out the fight unable to hurt him, but he will then slowly (or quickly) be able to get the upper hand by heating himself up beyond his limits.
We know from Sanji's fight with Queen that the heat of his flame depends on the strength of his haki. As long as Sanji's haki can withstand it, he can keep raising the heat.
We saw that GB is fire resistant but not fire proof. Raizo's flame technique had 0 effect, while Momo's boro breath broke his defenses handily.
So Sanji will start out the fight unable to hurt him, but he will then slowly (or quickly) be able to get the upper hand by heating himself up beyond his limits.