It is superior (in one sense) - it gives capabilities lacking in lower IQ people but it comes with metabolic and development time costs so you only get as much of it as can be translated into more surviving grandchildren. Same with any capacity like breath holding ability.
Sure, but that's the mechanism; select for genes that could (in theory) give a much higher IQ because the optimal point that balances costs and benefits is lower. Then if things all line up you randomly get a high outlier. The average is about where the optimal average is.