I spent five years on a PhD programme at an elite institute in one of the top four research universities in the UK. Some of the graduate students there were genuinely brilliant. But some couldn't think their way out of a wet paper bag. They still got PhDs, though, because they did everything their supervisor told them to.

Since leaving academia for industry, i have met a few more genuinely brilliant people. But they don't have PhDs, because that's not what you do in industry.

You can't make inferences about people's intelligence from their possession of PhDs. All it tells you is that they are capable of sustained hard work in trying conditions and on low pay, given a sufficient supply of coffee.