For this particular person, who wants to feel non-lonely and combat hubris while continuing to primarily do what they love (coding), I would say a heuristic I might use would be "developers who humble you with their abilities, and have great attitudes". Raw intellectual horsepower will likely be a big part of that, but I agree that "smart" alone is underspecified. For example, I think this person would enjoy working with other developers with high output at a low defect rate, with additional strengths in architecture.

Different people in different situations should absolutely look for different things in their teammates.