If the control group were doing nothing for eight weeks, they would expect no change. An expectation of change can illicit substantial differences. That's the placebo effect.
When administering medication and checking for results, the control group is given a sugar pill rather than nothing. If they were given nothing it would not eliminate the placebo effect.
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.
When administering medication and checking for results, the control group is given a sugar pill rather than nothing. If they were given nothing it would not eliminate the placebo effect.