It makes sense to me at least. People are most creative when they are out of their comfort zone and seeing the world from a slightly different perspective. Whether that's being unusually happy or unusually sad, any emotional condition that changes point of view can lead to ideas that wouldn't normally occur.
When sad, though. I'm a zombie. Nothing inspires creativity, nothing makes me excited. When I feel like that, I'm definitely not in my comfort zone. If you want help with your research, PM me and I'll be happy to share my experience with you.
On the other hand, this article seems to agree with the focused part, so I think the difference is probably how they defined creativity, as in this one they asked artists to do it while in the other article I believe they looked at brain patterns.