As I've read it, Froome may be asked to reproduce his physical state at the time of the "adverse" test and tested to see if they can reproduce the result.
September 7 was stage #18 of 21, a hilly stage following 2 mountain stages, a rest day, a time trial and another mountain stage. Good luck with reproducing that under trial conditions...in the lab or outside.
Edit: Just to make a difficult situation worse...it seems that there are personalities getting involved (link):
...McQuaid lost the acrimonious 2013 UCI presidential election to bitter rival and former British Cycling chief Brian Cookson, who was at the helm at the time of Froome's abnormal drugs test on 7 September before being replaced as president himself two weeks later.
McQuaid believes Cookson would "absolutely" have been told about the case.
...
McQuaid was among those heavily criticised in a landmark report published in 2015 into the sport's troubled recent history, while Cookson questioned the Irishman's handling of the Lance Armstrong scandal.