It was not established. An IQ test was never mentioned or presented to court. Some people shared their opinions. None said she was "highly" intelligent. The judge just said she was intelligent along with other descriptors.
From what I saw in court, she was very articulate, which is a form of intelligence, but she was not intelligent overall. Some of her behaviours and the things she decided to say bordered on stupid.
Also, she was unable to come up with realistic or feasible explanations, when so many of her online supporters were able to come up with excuse after excuse, and innocent reason after innocent reason. After seven years of thinking about this, she had hardly come up with anything at all for any aspect of her defence, which indicates both a lack of creativity, and a poor ability to understand how other people think.