Okay. So what does it mean when somebody says that LLMs are "low IQ"?

That if you gave them a test far outside their trained materials that it will do terrible compared to a human.

How do humans usually score if you test them far outside their trained material?

Poorly, which is why IQ tests are sometimes regarded as racist. Say the test asks a question about a regatta, and your culture has never introduced you to that word, so you fail that question, dropping your score by 20 points. Is that then a fair assessment of your IQ? Another place we see this is math ability. Street kids with no formal training in math may not be able to do traditional word problems, because they've never ridden, eg a train, but when those word problems are phrased in terms they are forced to use to survive, eg the prices of the trinkets they're forced to sell, then they excel at multiplying numbers in their head.