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.
An IQ test only determines a very specific type of intelligence, usually mathematical/linguistic. I know becuase I am a member of Mensa and I still dont believe that makes me anything other than good at IQ tests.
But LL does have a degree, various nursing courses and as you say was very articulate. She was consistently able to fool a lot of people, some very senior, for a number of years. Hardly the behaviour of a 'dumb' person.
Lacking creativity doesn't make her stupid. Knowing she is actually guilty there are very few feasable explanations you could come up with without them being easily disproven in court. Whatever explanation she had, would have to be part of her defense case so if it wasnt air-tight it wouldn't have held well in court. A million trolls coming up with some made up stories doesn't make them smarter than her. They didn't have to prove those stories in court.
An intelligent person can make dumb decisions too. I am sure you have like I have. My point was that her worst decisions were due to her narcissism, and not because she was really stupid.
A judge referring to her intelligence on sentencing her, after months and months of watching her in court is a good parameter to establishing she must have had some wits.
In addition, various psychiatrists have alluded to her possibly being a narcissist. That of course is in the absence of a formal evaluation, but you only have to see her general behaviour and demeanor in interviews and court to know there must be some truth in that.