That isn't a guarantee that 600 people will be born with both.

Obviously there's no guarantee, and there's also the fact that we may not have every case documented, but if it is true that we have 0 case documented, that's pretty far from 600. Even if it's not "impossible", then at the very least it's less likely.

It's a simple binomial probability calculation. The probability of one or more blind schizophrenic people born in the US, assuming they're independent variables, is 1-0,999998^330000000. I don't have a calculator on hand that can calculate that, but it's more than 1 - 10^-87. So the odds that there is no link between the two is close to the odds of guessing a specific bitcoin wallet's key in one try.