Dunno. We often saw CoA strikes emitting that same Haki. ACoC to me seems generally thicker and bigger, including Luffy's ACoC punches even if they touched, leaving a large jagged trail behind. We also see this lightning emitted with normal CoC when an attack connects.
On the other hand it's true that Luffy's CoA punches specifically didn't leave trails behind in the past. Oda failed to make a clear distinction between CoA and CoC.
To be honest, before Wano there are cases of Luffy causing black lightning when using hardening but ever since Wano started Oda has been extremely consistent with how his MC’s COA outside direct head-on clashes (that have their own visual effect), be it basic or advanced is depicted visually, here a compilation of pretty much all the moments Luffy used COA since the start of Wano :
Limb simply turn black. This has been consistent for basically 200 chapters by now.
I understand COC/COA discussions for secondary characters that are unconfirmed COC users as Oda drew the likes of Ulti and Jinbe black lightning,
But Oda has been keeping it simple for his MC for the last 5 years. Luffy uses COA : blackened limb only
Luffy uses COC : black lightning.
i don’t see any reason to believe he suddenly broke this pattern when he was still following it when he drew Luffy fighting the seraphims 15 chapters ago.
The explanation is likely that it is in Wano that Oda finalized how advCOC would look like and that Luffy would get it against Kaido. So when it comes to his protagonist whose abilities should be clear even for kids, Oda made the distinction between the moments he uses COA and the moment he uses COC clear.
I get why despite all of this one would be skeptical though, even as an admiral fan I never expected Kizaru to block an advCOC with one arm and not using visible hardening to defend himself.
But I’m confident, not because of agenda but because believing Luffy not using COA here is just believing Oda is following the pattern he has been consistent for several years by now.