This is not true, see for example WADA's own statement:
"populations of both males and females. It should be borne in mind that there is significant variation between individuals . A normal level for one individual may in another be elevated and be consistent with doping."
Also there are known conditions which substantially reduce the level of epitestosterone and therefore increase the ratio without any doping taking place. This is one reason the T/E ratio test is now only used as a screening test and further testing is required for an adverse finding. Although previously others had their careers ended by this test in the past who probably wouldn't under the current rules. Also sample degradation is possible, particularly if exposed to high temperatures such as occurred in the Lagat case. That the testing authority thought it was appropriate to transport samples unrefrigerated in the trunk of a car for 18+hrs in 40ºC weather to save a few bucks rather than ship it properly was a disgrace! Again following WADA's own documentation:
"The urine Sample is not collected under sterile conditions, and where the circumstances are favourable, the microbes present in the Sample can cause changes to the profile of the urinary steroids. Initially there is cleavage of the glucuronides and sulfates followed by modifications of the steroids’ structure by oxido-reductive reactions. To report an Adverse Analytical Finding of an
elevated T/E value, testosterone or epitestosterone concentration or any other endogenous steroid parameters, the concentration of free testosterone and/or epitestosterone in the specimen is not to exceed 5% of the respective glucuroconjugates. Elevated amounts of 5a- and 5ß-androstan-3,17-dione in the free form also indicate microbial degradation."
Highly concentrated urine such as might be obtained from a dehydrated athlete could be more subject to this post sampling modification.
They rely in part on this method now but it can so easily be defeated that I would hope that they're looking into an alternative.
Sorry I'm not convinced. The science is good but it's not infallible, particularly when WADA don't always follow their own protocols and as indicated above sometimes don't use best practice for their sample handling.