1. That article includes dead people.

2. There's no indication of how, exactly, they measure intelligence.

3. The article does not even claim the people listed are in in order of intelligence.

4. And there is in fact no way to rank people's "intelligence" anywhere near this exactly.

Conclusion: to conclude from that article that Grigori is the "fourth smartest person" is not smart.

I might add that Perelman is pathologically cut off from the world and seems to have no contact with any humans at all. That isn't a sign of good mental health or adjustment.