I had the same confusion after seeing the headlines, but after reading the respective articles, I think the point is that happiness and non-focussed dreamy stat

I had the same confusion after seeing the headlines, but after reading the respective articles, I think the point is that happiness and non-focussed dreamy state are helpful in freely exploring problems and links semi-consciously – sort of finding ideas in the back of your head – while the sadness (and anxiety) is needed to focus and keep trying to succeed in hard disciplines. Moods correlate with states of focus and attention, and hence some states of mind are more useful in particular phases of the creative process. I think it makes sense to some extent, as a happy person has no needs and can let their mind float aimlessly while an unhappy person wants to strive, focus and achieve to change his/her condition or to at least forget about the state of mind he/she is in.