Sorry for missing the post where you already explained it. My bad.
Anyway…
Well, I explained why I believe so in one of my posts since then, but basically because of the way he used it and the way it has been identified. Oda doesn't leave us to assume Luffy has used it dozens of times without any of us knowing, and he hasn't hinted at Luffy having a leak of some sort. When Oda wants Luffy to use it, he makes it obvious. Therefore, Luffy has never used it in front of Blackbeard.
I think that is very, very presumptuous. Just because Oda hasn't clearly hinted at something doesn't mean anything at all really. Absence of evidence is not evidence of absence. An annoying saying, but i think it applies here.
And Luffy surely has some sort of leak. How else would you define his unintentional outbursts? It makes perfect sense to say they are all instances of his latent abilities in Haki "leaking" out.
And to say Oda always makes it obvious when Luffy uses Haki is an unprovable and pretty baseless statement imo. All we know for certain is that he makes it obvious when he wants us the readers to understand what just happened. Oda is not limited to using Haki in a blatantly obvious and instantly clear and understandable manner.
Furthermore, Luffy has been emotional dozens of times around people who can use the ability, and yet noone has made a comment until Amazon Lily? How come when Luffy was emotional dozens of times on Amazon Lily nobody took notice until he actually used it? The simple answer would be that it isn't something you can see or naturally sense, but something that, when used, can be felt or identified. It goes without say that from there, of course an island of people who know all the ins and outs of the ability can identify it immediately when it is used. So, again, despite Blacky's usage of the word having no contextual significance were he referring to the ability, it simply wouldn't add up given…a good many things. Plus, that fate thing needs to bite him in the ass so to speak eventually.
But how do you know Luffy wasn't somehow "using" haki in both encounters with BB? In both instances he was in competitive fighting spirits. It would make prefect sense if, even when not imbuing attacks or using it bursts to knock people out or whatever else, but just when Luffy's ready to throw down, the Haki somehow makes itself apparent.
That Boa or others who might be able to detect such things haven't commented on it (until it was something truly unexpected, at least) doesn't really have to mean anything. It could easily just be Oda picking and choosing his spots to reveal more about haki/get people thinking and talking about.
Or that could all be totally way off the mark Bullshit. I don't and can't know, and neither can you. That's my point here. There's any number of possibilities that have BB's comments directly relating to Luffy's Haki the technique or ability or whatever, instead of his personal ambition, his confidence or his willpower or any of that. It makes no sense for his words to be referring to the latter. Especially considering that BB is in no place to comment on intricate aspects of Luffy's personality when the guy has spent a grand total of like 5 minutes in his presence. All of those aspects of Luffy's personality are pretty much exactly the same now as they always have been anyway.
I just don't see how it makes any sense to assume Blackbeard wasn't referring to "Haki the technique."