Functional yes, but an IQ of 84 isn't "slightly below the normal range", it's the 14th percentile. Not to say that it's not an achievement with just 10% of a brain, but he wasn't an average intelligence person, he likely struggles with a lot of things.
This is really interesting from the perspective of gradual replacement/mind uploading: what is the absolute minimum portion of the brain that we would have to target?
Understanding this could probably make the problem easier by some factor (but not "easy" in any sense.)
I was going to write "I don't think this specifically is where we need to look", but then I remembered there's two different reasons for mind uploading.
If you want the capabilities and don't care either way about personhood of the uploads, this is exactly what you need.
If you do care about the personhood of the uploads, regardless of if you want them to have it (immortality) or not have it (a competent workforce that doesn't need good conditions), we have yet to even figure out in a rigorous testable sense what 'personhood' really means — which is why we're still arguing about the ethics of abortion and meat.