I'm by no means an expert, but from my reading of body hacking and dieting etc, it seems that higher intensity workouts lead to increased cortisol release, a hormone released in response to stress. I'm not sure if this is the hormone they're referencing though, because IIRC extended periods of elevated cortisol levels are associated with Parksinson's/Alzheimers...
From what I understand, high-intensity workouts do raise cortisol but only temporarily. Long-term, exercise-induced adaptations counteract those raised cortisol levels, and actually lower them.