That is not an answer, but another presumption. The source would be something specific, like nandrolone precursor, or vitamins/supplements contaminated during manufacture, or consumption of edible parts of intact boar, or beef injected with nandrolone.
Out here in the real world, outside the artificial context created by WADA for anti-doping, it is impossible to conclude she intentionally doped with a banned product for performance enhancing reasons without first establishing the source was a banned product.
I think it is you who does not understand how this arbitration proceeding worked.
She did rebut the presumption, but the CAS subjectively ruled that her rebuttal didn't meet the required standard, lacking necessary "specific and concrete elements". This is well known to be a difficult proof to provide.
My expressed opinions about the injustice of cases like Houlihan's is narrowly limited to cases consistent with accidental ingestion, e.g. from eating meat known to contain banned substances. Houlihan's low values and CIR in the endogenous range, are consistent with ingestion of intact boar, creating an ambiguity that cannot be resolved by arguments based on national statistics, or presumptions of guilt until proven innocent. But if, for example, her result was 300 ng/ml (twice the amount Prof. Ayotte published in the literature), and/or the CIR was -27‰, clearly exogenous, these would not be consistent with ingestion of pork.
Note that anti-doping bodies were always able to punish athletes for unintentional doping. If Houlihan had established non-intent, she would still be looking at a 2-year ban. This is what routinely happened before 2015. In order to eliminate the ban, she would have had to establish no (significant) fault or negligence.
"Strict liability" in anti-doping means that the athlete will be found to have violated rules, regardless of fault, negligence, intent, or knowledge. The athlete is responsible not only for their own conduct, but also the conduct of others they choose to associate with, e.g. spouse, coach, staff, in cases of sabotage, and apparently the complete supply chain of all the meat they eat and supplements/vitamins they take.
Even if an athlete is able to prove no-fault, no-negligence, non-intent, and lack of knowledge, they will not be acquitted -- it is still considered a rule violation. This is explicit in the Code. The next violation will be considered the second one. What happens is that the sanction for the guilty verdict can be reduced or eliminated, but it still counts as a first strike. They also cannot recover lost time served under a suspension, lost prizes and sponsorship, and legal, investigative, and scientific expenses.