I've been doing a lot of research on creativity lately and the consensus seems to be that any strong emotion can make a person feel more creative. Joy or depression, love or hate. It seems contradictory and people prefer clear messages, which is why I'd imagine that the authors of the respective pieces decided to present only one side or the other.

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.