Personally my money is on PoL
Oda introduced that ability into the series for a reason and really didn't need to have Sanji use it not once but twice on Egghead.
Could have simply just had only Kuma display it if he simply wanted to show the characteristic ability of the Buccaneers.
Moreover two important tings to also note are
① since the TS even though Sanji's haki has obviously been progressing with each arc, there hasn't really been as much emphasis on it as Luffy and Zoro but rather Oda has chosen to focus on his lineage factor instead so suddenly switching to ACoC isn't really an organic way to go about Sanji's strength progression.
② there's always some sort of foreshadowing for Sanji's PUs
- prior to Ifrit Jambe Oda showed Sanji's unusual resistance to heat. We saw this in his fight against pearl when Luffy's ass was burning from simply being in the vicinitty of Pearl's flames whereas Sanji was casually dancing around in it.
- prior to hell memories we got that one scene with Sanji's whole body combusting on EL when he found out Absalom saw Nami's naked body
- prior to the exoskeleton, we've seen Sanji crack his bones several times in the series with the most recent being his fight against vergo
*Now I know folks would argue that Sanji has shown attributes of being a conqueror prior as such it'd be inline with this pattern but imo PoL which was introduced last arc is the more organic way of going about Sanji's development
Also as an add on, there's been quite a bit of emphasis on "developing what you have" this arc, with Gaban telling Luffy to engrave the proper utility of CoC in his mind and then Gaban scolding Zoro for not even knowing he has CoC, so with this "development/buid on" theme, Sanji building on CoC which he's still yet to manifest doesn't really fit (though this might simply be me reading too much into this).
On CoC, I still think Sanji has a strong chance of unlocking it. However, when he does I feel it'd be more of supplementary power up to something else. I'm still of the opinion that Sanji could potentially unlock CoC with his next Jambe upgrade.
We already have 悪魔(Demon)/Diable Jambe 魔神(High Ranking Demon)/Ifrit Jambe so next up will have to be 魔王 (Demon King)/??? Jambe. That'd be a more organic way of going about him awakening CoC imo.
