I know but he trying to stop doflamingo cage with his haki was a bit L
It's plot.
I'm actually happy that scene exists though, cause the kanji/sfx when he coated his sword in Armament actually helped me figure out he was using Haki against Sabo and Sabo still resisted it and turned into fire anyway (which is good for agendas lol).
Sounds dumb, but look, I promise it'll make some sense:
Look at the bottom panels here:
And the top panel here:
See how he has purple lightning? That's cause he's using his DF and only his DF in that clash.
Now, look here
Make note of the kanji in the bottom left panel, it'll be important.
Now, besides that, see how the lightning is black and not purple?
We see this black lightning with other uses of Armament in Dressrosa, for example:
But that's not all. Remember I told you to remember the kanji in the panel where he uses Gravity Blade? That's because the raw and colored versions of these next two pages both have the exact same kanji (or sfx, what ever it's called).
We also often see the "Zrd" or "Zmm" effects when Haki is being used, we even see it being used for Zoro a chapter before that (
here).
Thanks for coming to my Haki class, stay tuned for the next lesson: Does Zoro have ACoC?