Why a Dark Whisperer might make such a basic error, Gu Tao wasn't quite sure, probably because of those fragments of destruction in his memory, just like how those who fear spiders tend to see them everywhere, and those who love sausages seem to notice them more easily, information about destruction was something that Whisperers paid attention to. So, perhaps it subconsciously thought that Gu Tao was an exceptional master, especially since it had just been defeated by him.
Whisperers aren't human, and it had never dwelled in the human world; its understanding of this world came from absorbing others' memories and dreams. However, dreams are intangible, and memories are chaotic, hence its logic became fixed into a strange pattern. Moreover, it had never encountered someone like Gu Tao, who, despite having already defeated it, was so seemingly weak.
But now that the contract had been signed, it realized it had been duped, majorly duped.