Interesting Catch.
I still believe there are levels to CoA.
Take for example Sai.
When Sai was in the coliseum he was capable of using the invisible haki, but when he finally unlocked the hardening(CoA) against his Grandfather Oda began drawing him using it in multiple scenarios i.e. against Lao G & protecting himself against Pica's attack.
So there does seem to be a level a difference, between the two, I could be wrong though.
I still believe there are levels to CoA.
Take for example Sai.
When Sai was in the coliseum he was capable of using the invisible haki, but when he finally unlocked the hardening(CoA) against his Grandfather Oda began drawing him using it in multiple scenarios i.e. against Lao G & protecting himself against Pica's attack.
So there does seem to be a level a difference, between the two, I could be wrong though.
There are 2 forms of the Invisible Armament Haki. The Black coloured one is invisible, but the strongest form of Armament.
As you pointed out, Sai was using the invisible one, and then unlocked the black shaded Armament that is also invisible.
Most Haki users are only capable of using the basic invisible one like Tashigi for example.