The big difference here is that the sentence is treating "a brilliant mind" as the person, not as one of many body parts within that person. By saying that Trevor "is a brilliant mind" they're equating Trevor with "brilliant mind" (as opposed to saying Trevor "has" a brilliant mind, which would make the mind one of his body parts).
A similar example would be "In a recent movie, Matt Damon starred as "The Brain," a scientist who..." Here "The Brain" is the name of a character, so you'd treat that as a person, and therefore the modifier "a scientist" (which can modify a person but not a body part) would be a valid modifier.
NOW...the great thing about GMAC is that it statistically tests all of its questions for cultural bias so if it were a case where people in one area of the world (here in the U.S. I think it's pretty common to see someone called "a brilliant mind" and to know that that term applies to a person, but if you didn't grow up with that euphemism I could see it being tough) get the problem right way more frequently than people elsewhere, they'll flag it and investigate before it would ever affect your score. BUT I'd be leery of depending on that, so the lesson I'd take away here is that that word "is" equates Trevor with "brilliant mind" so it's logical to use "brilliant mind" with modifiers for a person.