That person might build exceptionally good PC's. Judging the intelligence of the person purely on spelling & grammar in one email is a bit rash. Dyslexia is a thing.
I seriously doubt he’s exceptionally good at anything if he can’t be good enough at one of the very basic things that even kids can be good at. Life doesn’t work this way. There’s probably an exception somewhere, as you said he could have one of those modern diseases, but it’s not a risk worth taking for society. It’s not efficient this way. Like hiring women for military or police jobs. It’s not making the world better.