"It" means "potential". The very next page Ivanokov, who not only knows who Luffy's family is but definitely knows Haki himself, wonders where he learned to do it. Not to mention the fact that Luffy didn't start unleashing it until after he'd trained his entire childhood, and been through the first half of the grandline and after fighting super tough opponents. This isn't dragonball where they looked at Luffy the day he was born and went "A power level of two! He's worthless, send him to earth!"
If Luffy hadn't met Shanks and decided to become a pirate, he wouldn't have ever learned the ability.
It's not a Naruto bloodline trait. It's something anyone with enough arrogance, pride and self confidence, with the power to back it up, can eventually reach. Basic haki is something everyone can learn (every Amazon has it), but King's ambition has gone to a dozen different characters, few of them related. Hancock, Dofla, Chinjao, Rayleigh, Shanks, Whitebeard, presumably Roger, Ace, implied Dragon, Luffy? WHat do they have in common? A desire and drive to be the top and be better than those beneath them.
Garp does not seem to have it… and he's a guy that was always content to be just a Vice Admiral, rather than the very top.
It's drive and will and inspiration that unlock King's Ambition, not just how you're born.