Why not? 1,000 is enough to get a nice bell curve, and it doesn't sound like they were working with a huge population.
It doesn't really state in the article that the students listen to metal because of any inherent intelligence in the music, but because of its stress relieving qualities (i.e. loud, fast and angry). Look at the examples listed: Slipknot, In Flames, Dragonforce - they are basically listening to the most popular bands in the genre, not the ones that could be considered more intellectual than your average pop music selection by any great margin. The higher level of musical complexity in some of the selections (Tool, Dragonforce) is about the only indication of the students choosing the music based on their (or its) intelligence, but as such a reason isn't mentioned drawing this conclusion would be quite speculative. It is also important to note that these gifted children weren't exactly picking metal with overwhelming frequency - Rock, R&B and pop were *all* more popular than metal among them. Should we conclude that metal is less intelligent than these other genres because of this study? I think not, the article merely attempts to offer an explanation as to why *some* gifted kids listen to metal: because they have "lower self-esteem and more difficulties in family relationships and friendships."