1. Each biomarker has to be validated independently. 2. There's a huge difference between something used for testing, and something used for intervention. Grey hair is a pretty OK test for aging, but clearly dying it isn't going to arrest the aging process.

Long term degenerative conditions are some of the most difficult problems for such validation, due to time, cost, and the complexity of risk in real populations.

I don't know anything about how well the current study actually links to mitochondrial biogenesis, but I know that we don't have satisfactory interventional biomarkers for a process as complex as aging. By all means continue researching this, but I think it's eminently sensible to be skeptical at this point.