Personally I believe every Shichibukai has armament haki just like every VA does as both are being used to fight high level pirates. What good would they be if there's a good chunk of pirates they couldn't do anything against.
As to the show casing, we know it wasn't shown as clearly pre MF. But other explanations could be Moriah lost his will after losing to Kaido and since haki is tied to will his haki/will at TB was miniscule.
See earlier One Piece was heavily focused on DF and the ability it grants to its users. That's why a good chunk of the manga only showed different DF ability. It was like Post TS where every guy started having some form of haki.
But my point stands as guys like Crocodile never showed any haki. He had unlocked Awakening so that could be a reason he slacked in Haki department.
In case of Moriah I don't see how he completely forgot to use Haki even though he fought with Kaido. Will power explaination I m not fully convinced as we saw WB commanders continued using Armament even though they lost WB n Ace. So there situation should be no different then Moria who lost his crew.
Also I doubt Buggy got any haki n he was part of Roger who roamed in NW.
Again I m sure it was never said that in NW pirates have one form of Haki. Some of the flying six members didn't showed any haki, Robin who spent a good chunk of decade hopping from one pirates ship to another never showed any haki neither did some of SHs commanders.
I agree if one has to survive in NW then some form of haki is necessary but we should not forget that In NW it's more like become underling of a Yonko and you kinda get protection from them.