LUO JIN arrived at that conclusion because of the mannequin that released green smoke and those vials that floated around Lovelee. Not to mention, the corrosive property of that vial with black liquid in it.
Alchemists belonged to a special class alongside Blacksmiths. The former could create potions while the latter could forge weapons. They were the only classes that could set up shops and sell whatever they made to other players.
The keyword here was the word 'creation'. The things that Lovelee had shown so far, as well as the things she had said, showed that she created these things that she had used to attack him. From that mannequin to those vials.
After that realization, it came off as a surprise to him. After all, Alchemists and Blacksmiths were not known to participate in events like this. Heck, they didn't even have their own PvP ranking. Because most of them were known for doing business instead of really playing the game.