Placebo might be a fair critique if we were talking about people's self-reported stress or well-being. Here we're talking about physical changes in the amygdala.
The brain and nervous system changes based on it's use and function. Look up neoroplasticity. This is nothing new. A placebo could certainly elicit a change within 8 weeks if participants thought it was helping them cope with stress. If you think dancing clockwise in a circle for ten minutes a day will help you, it probably will. That doesn't mean there is anything useful in the practice.