I think the author is saying that there isn't much difference on a relative scale. For example, humans typically weigh between 100 and 300 lbs and have a certain range of strength (say, deadlifting between 50 and 1000 lbs). Compare that to an elephant or a mouse and you can see that humans represent a very small range of weight and strength, even if it seems like lifting more than 1000 lbs is impossible in the same way lifting 10 lbs would seem impossible to a mouse. Similarly, we see intelligence on a subjective scale where the smartest humans seem brilliant. But we have nothing to compare them to. If another species (or artificial intelligence or alien) that was vastly more intelligent than us came along and compared human-level intelligence to its own then even geniuses would seem simple and stupid. There would be effectively no difference between the smartest humans and dumbest humans.