Though it's more like a proof that Oda was far from having properly planned for armament haki at this point.
Yeah, it seems like Oda already had the Idea for conquerors and observation haki with Shanks and Skypea (Enels Mantra) very early on, and Armament came later into the mix.
But, we had an alternative concept or Idea for armament which sounded promising in order to fight these Logia Fruit Users that seemed so invincible early on. During the Alabasta Arc, we had the Sea Prism Stone introduced to us, in an arc where 3 different Logia users were present. Ace, Smoker & Crocodile.
I remember when I thought this would be the thing to defeat crocodile back then. But then I also wondered could you even hit them with it when they are intangible? Smoker had a clash with Ace, and the conclusion was that a fight between their Elements would never come to an end. Wouldn't it have worked if he attacked Ace with the tip of his Weapon?
In a later chapter we see Zoro, Usopp, Nami, Luffy and Smoker trapped in crocodiles prison cell made out of sea prism stone as well. And smoker himself said he would have escaped already by turning into smoke if that wasn't the case. Which sounded a little too convenient to me, because when you look at that Scene, that Prison had many holes in it from where Smoker could have escaped if he turned into Smoke without touching the cell itself.
Unless the Sea Prism emits an Aura so strong that it would have stopped him nonetheless. So, even in his intangible Form, it would have affected him. That means Smoker could have hit Ace with his Weapon, which would be similar to hitting someone with armament haki.