Chun didn't know the principle behind the Purification Technique; it apparently required contact with water. A layer of glass in between probably wouldn't work, so he filled the bottle with water without capping it, using his palm as the lid. In this way, his palm was still in contact with the water, and the throwing posture was very standard, much like pulling the safety pin and throwing a grenade. It wouldn't distort the throwing technique for splashing water either...
Chun raised the bottle, and the purification started—at that moment, he suddenly noticed the water in the bottle stirred by a vortex from his palm and emitted a faint glow—that glow! He'd seen it before when purifying the poison dust of the bat swarm in the Water Prison but hadn't paid attention!
Chun just stared for a moment, forgetting to throw. When the vortex weakened—system prompt: You have activated the Purification Technique, cleansing impurities from the water in the bottle.
Holy shit!