Sure thing, although I would argue the increased evolutionary fitness would be very small. The fact that they roll in it vs just inhaling it to me also suggests this.
Maybe their use of the word "functional" is different than mine, and we're saying the same thing. But to me there's a big difference from saying e.g. "Our findings suggest that chimps using sticks to dislodge food is a functional behavior."
In evolution, couldn't it be that the trait of enjoying catnip made them more resilient to mosquitos and thus had a higher chance of surviving, or so?