You misunderstand (or rather, I didn't really explain myself well). I'm not saying that isn't what it is, but rather that it's simply the effect Haki naturally has on rubber. Because, well, it's rubber. Haki normally hardens limbs or objects or what have you, but in Luffy's case his body is made of rubber, so it reacts a little differently when he "hardens" it. I would find this, the idea that Haki interacts differently with different DF powers, more appealing than Luffy having both that and the "generic strength boost" thing. And it just makes a ton of sense for them to be one and the same for the reasons I outlined in my previous post.
I'd say that could plausibly fall under "Oda keeping it hidden at the time".
We've already seen with Eneru that haki can have a unique effect when paired with a DF. Since Eneru had the lightning DF, he was able to enhance his CoO (mantra) to be able to hear the entire island of Skypiea. For Luffy, since he has a Rubber DF, when he applies CoA to it. It gives off a property similar to vulcanization and harden the rubber and giving it a black look. I expect now that Oda has revealed haki that haki DF users in the New World will use haki in unique ways combined exclusively with their DF powers.