pass the basic didn't mean it needed to be a hig-level haki. I mean if Oda plan on giving him such level in haki, there was no reason to make Rayleigh say that 2 years would be kind of short to learn even the basic.
Basic use of haki doesn't equate to low amount of Haki. Even with knowing just the basics the amount of Haki Luffy has is a lot.