Did Spirit Cat have an auto-targeting system?
Of course not. The summons of a Summoner, when set to free attack mode, would automatically attack any non-teammate target that wandered into its aggro range. Spirit Cat and a comparatively larger aggro range, more or less meaning that it's senses were sharper.
This map was carefully chosen by Luo Ji. There was no other one on one map that he was more familiar with. Jia Xing had thought that he would be able to sneak up on Luo Ji just by taking a more roundabout route, viewing Luo Ji as a complete newbie and underestimating his meticulous mind and familiarity with the map.
A sneak attack from behind was something Luo Ji he had accounted for! Spirit Cat had been arranged in a position where it would be able to sense if any enemies were in that area. Poor Jia Xing had no idea, happily waiting for a chance to strike. In the end, what he got was a Spirit Cat, and then another Devil World Flower.