I think intelligence is not fixed. I think that beyond a certain point, it comes down to hard work. In my math program we had a handful of bright people from Eastern Europe and China. They were clearly talented mathematically and had been singled out back home for extra preparation. They were clearly ahead when we started the Real Analysis/Abtract Algebra classes, but after the first few months, everyone caught up. The pace of work was so hard and we all worked so hard and lived and breathed math that they were soon in the middle of the pack. In later classes where you had to do original thinking and grunt stuff out, they were no better than anyone else.
Intelligence is pretty fixed. Skill level is improved with the right experience.
A lot of things come down to tacit knowledge that are separate from native intelligence. The only way to learn certain things is through experience. Not by book.
For example, in this bear market in the stock market, as opposed to the last bear market (2007), I was able to short the market at a much more appropriate time because I have been paying close attention to it for the past few years and I know what to look for now having absorbed a much greater amount of information.
My intelligence didn't change (in fact, it probably declined with age). But my market timing skill got better.
