I'd venture to make a broader claim. It's not so much that relaxation affords for higher incidence of creative breakthroughs as it is that relaxation is a type of activity that supplies the human brain with a steady, stable flow of blood, and hence, oxygen. Essentially, it's good circulation that gets those creative juices flowing.

I think if we look at this physiologically, we begin to see a connection between seemingly incongruous activities that we could all attest to being potential hotbeds for insight.

Relaxation and meditation are certainly activities that predispose us to steady breathing, but they are not the only ones. Consider strenuous activities like exercise and sex. Oftentimes, the breakthroughs here follow directly after the activity while falling back to a rest state.

Finally, consider periods of sustained intense activity like marathon running. I have a close friend who runs them regularly who has remarked that she gets some of her best ideas during her runs. I'd speculate this happens when the runner's body has adjusted to a certain pace and breathing and blood flow has stabilized.

Just another angle to look at this, I guess.