Sounds like maybe they put the cart before the horse. "Intelligence," of the kind that can be scored on academic tests, is a combination of raw potential, opportunity and effort. You can have amazing potential for intellect, but never get off the family farm - get "home schooled" and told that the earth is flat and math is a tool of the devil - you will then fail any external metric of "intelligence" that requires you to know and apply facts. Likewise, someone who has barely average potential can, given adequate opportunity and effort, develop the skills and abilities to do well on academic tests or external measures of "intelligence."
"Intelligence" as measured by academic performance would thus not correlate well with raw genetic markers (i.e. raw potential). First they'd need to define what capacity of the person they want to deem "intelligence" e.g. - are we talking logical deductive capacity, ability with mathematical computation, the ability to communicate effectively, problem solving skills - what is "intelligence?" Second, they would need to find some way to test that capacity that isn't commonly understood and used outside the study, so, for example, not asking participants to take a math test because then the opportunity and effort elements interfere with the measure of raw capacity - you'd need a test that challenged the ability to learn and comprehend without falling back on existent structures; this would be very difficult to formulate well, and any test taker could only use that test once. Third, they'd need to look for common physical markers in the developed body / brain for those that demonstrate whatever capacity they've defined as intelligence - e.g. nerve density, size of the brain cavity, etc. Only then can you look for a genetic source for that physical marker.
"Intelligence" isn't height or weight or eye color - it is too subjective a quality to seek out before defining your term very narrowly.