Charlie already knew when he fought Barlin that he couldn't hide his cultivation.
Ruby had taught him the technique to hide cultivation, but that was only if he didn't use Reiki in the first place, and managed to hide himself.
But now, Barlin found him, so such deceit was pointless.
However, that also meant Barlin could see his Reiki's state while he kept popping pills, so why not show off and taunt him further?
Charlie's words left Barlin fuming, but it also made Barlin take his fight mor