The Chilean first-hand testimonies make the dog-rape plausible, significantly enhance the probability of the Nazi-era claims.
You are ignoring the possibility that the Chilean female prisoners who claimed to have been raped by a dog simply invented the story.
In order to make a reasoned and impartial assessment of the claim we would need to know more about the person or persons making it. Were those persons making it entirely innocent, non-political people who had simply got caught up in the repression imposed by the Pinochet regime?
Or were they committed political opponents of that regime? In the latter case, they cannot be regarded as entirely reliable witnesses since they would have had a strong incentive to exaggerate and even fictionalise the harsh treatment they received from their captors, in order to portray them as exceptionally inhuman and perverted.
Another suspicious element in the stories is the inclusion of the claim that rats were introduced into their vaginas and then induced to claw them by being given electric shocks. That is very obviously derived from the "Chinese rat torture" described in George Orwell's novel "1984". Orwell most probably derived that description from the book by Sergei Mel'gunov, "Red Terror in Russia", which was published in the West in the early 1920s, and included all sorts of lurid claims about atrocities committed by the Bolsheviks (including that they made gloves and other objects from the skin of their victims; the book even contains photos of those alleged objects).
According to Mel'gunov, the "rat torture" was introduced by Chinese mercenaries working for the Cheka. In his description, one end of metal pipe is securely fastened to the prisoners face; rats are introduced into the pipe from the opposite end, which is then heated so that the rats cannot escape through it. The rats then try to escape through the end attached to the prisoners face, and finding it blocked, they begin to bite and scratch at the prisoners face in their desperate attempt to escape from the pipe that is heating up and causing them pain.
It is noteworthy that many of the alleged Bolshevik atrocities described in Mel'gunov's book in the 1920s were later attributed by Communists who had been prisoners of the Germans to their anti-Communist captors, eg the allegation that those captors had made objects out of the skin of prisoners. The reason why the Communist ex-prisoners made those allegation is obvious; they were projecting a lurid allegation that had been made against them back against the people who had made it. It was a case of "it was you who did those horrible things, not us".
If the Chilean prisoners making the allegation of "rat torture" were in fact Communists, then they would have had a strong motive to manufacture that claim against their anti-Communist captors. In the first place, they would probably have been aware that "rat torture" was an allegation that had been made against their fellow Communists in the past, and they would have had the same motivation to turn that allegation against their captors as the Communist former inmates of the Bergan-Belsen concentration camp had had to turn the allegation of manufacturing articles from human skin against their former captors.
The "dog rape" element could well have arisen from the fact that the women's jailors did have dogs that may well have been used to intimidate prisoners, and perhaps even attack them physically by biting, something that is fairly normal canine behaviour. In that case, a normal biting attack by a dog would have been fictionalised into a sexual attack, in order to make the jailors appear even more inhuman and perverted than they actually were.