When facing Xinghe, which represented a higher civilization, the Serpent People really lacked confidence and conviction.
Does high intelligence necessarily equate to deception?
This question does not need to be investigated in detail. Einstein's intelligence was certainly high, but that doesn't mean Einstein was particularly good at deceiving people.
Moreover, when a civilization has developed to the point where it can technologically overwhelm any species it encounters, they naturally devote less research to strategy and tactics.
Therefore, the Serpent People's intelligence is definitely not low, but they are indeed very easily deceived, very easily bamboozled by Xinghe.
Now Xinghe has learned some bad habits; she has been culturally influenced in reverse by Earth and has learned how to deceive. However, it's clear that she hasn't mastered the essence of deception yet, or to put it another way, she still has much to learn.