The one thing I truly dislike about studies like these (and most animal studies) is the assumption that all cats are created equal.

Because 15 individuals obviously will account for a species covering nearly the whole globe.

Good studies on humans take into account economic status, upbringing, environment, etc.

All the same elements can be applied to animals. All the things that can degenerate intelligence (neglect, abuse, etc.) or increase intelligence (games, interaction, social environment) in humans do the same for animals.