Shelby is clean as a whistle. I don't see BTC getting popped for even slight indiscretions similar to the Oregon Project and everyone knows that Jerry Schumacher is the best professional coach in distance events.
First off, Shelby is an all-time talent. Look at all her high school achievements and then what she did as a Sun Devil. Once in a generation for a state like Iowa.
Second, everyone knows that she was low mileage. She was originally an 800m runner. ASU moved her up to the 1500 and then got her in cross country shape. She was the PAC 12 champ her senior year beating a very deep field and was a multiple time All-American. Now she's probably doing some bigger mileage with Shalane and Emily Infeld at the BTC.
Third, she's mentally tough. Shelby comes from a small town in Iowa where the weak do not survive. Survivorship bias comes into play here.
Fourth point, her biomechanics are near perfect and she doesn't have a lot of the unnecessary crossover motion in the upper body that weaker runners suffer from. She's gotten stronger with BTC and will continue to dominate.
Lastly, she's never trained in a country (won't mention specifics) or with a group of runners who are constantly associated with doping.