Sanji used Conquerors Haki here without realizing it
What Gaban told him is that selfless guys like Sanji have the most trouble becoming Conquerors Haki users because it only comes surging out of them when the people they love are in danger, and not themselves.
Sanji will have to learn how to tap into the emotions that unleash his Conquerors at will if he wants to keep up with Luffy and Zoro