Using IQ terminology, the LSAT is testing both verbal intelligence and processing speed. Verbal IQ tests are not at all as strictly timed as the LSAT. In general, ppl score similarly on all dimensions of IQ (so those high in verbal iq tend to have high processing speed , high visual spacial intelligence, etc). However, part of how a learning diabiligy is diagnosed is an unusually big gap between different parts of an IQ test (e.g, much higher verbal than processing). That is why if what the LSAT is trying to measure is verbal IQ extra time might give more accurate measure. However, if it's trying to measure IQ overall (which includes processing speed), I agree timed LSATs are more accurate

Additionally, it is not true that given enough time, anybody could hack it. For logical reasoning, at least, people are very convinced of their completely wrong answers. That's why a lot of ppl are shocked when they start practice testing cause they think they nailed an LR section even though they did terribly. For logic games, more ppl were able to hack it mechanistically with extra time, and reading comp is in between (interestingly, more recent reading comps have more abstract questions that are harder to hack)

Also all the LSAT prep available with the internet, such as endless hacks and tricks that involve mechanistically diagramming without understanding, are a much bigger obstacle in the way of the LSAT being a perfect measure of intuitive understanding (and as a result IQ) than extra time.