Here is what you don't understand:
It shouldn't be anyone's task to evaluate every which scenario that the nandrolone could get into Shelby's system besides Shelby and whoever she hires. They are tasked for coming up with a theory, defending that theory, and then proving it above 50% probability. Shelby had ample resources and time, and she came up with the carne asada burrito that she ordered being a greasy, half-eaten pork stomach burrito that she got instead with meat from an uncastrated boar. That, and solely that, is what is to be evaluated.
Now, if you want to evaluate Shelby in some other alternate universe, go nuts. Maybe, in the most generous interpretation of this she had a tainted supplement? Maybe she had an off-menu pork kidney/heart burrito from an uncastrated boar that was missed in a pork plant somehow and she missed itand the food truck missed that were cooking pork kidney? But she herself has not argued these things, so you're just throwing up a shot in the dark.
I had to sit on a jury in a criminal case once and we had one guy who kept inventing up "what about this scenarios" for the accuser because her story just didn't add up. Just like him, I would again remind you that is not the job for whoever is making the ruling here.