Still think it’s haki related, like targeting the souls for example
It's haki related imo but I don't think "simple" aCOC blocked the regen for two reason
1)It would make Egghead writing one of the worst writing in fiction and this is not a hyperbole. When you read shonen you should have a tolerance for PIS but in this case Luffy literally complained 2-3 times about how he couldn't damage the elders no matter what he tried. Like that would legit make me quit any serious discussion concerning the series
2)When Gaban cut Sommers hand and first blocked his regen there was no haki lightning