"Supposedly the only evidence, "the postwar testimony of individual survivors."
First of all, consider the implicit conspiracy theory. Notice how the testimony of every single inmate of every Nazi camp is automatically dismissed as unconvincing. This total dismissal of inmates' testimony, along with the equally-total dismissal of the Nazis' own testimony (!), is the largest unspoken assumption of Holocaust-denial.
This assumption, which is not often spelled out, is that the attempted Jewish genocide never took place, but rather that a secret conspiracy of Jews, starting around 1941, planted and forged myriad documents to prove that it did; then, after the war, they rounded up all the camp survivors and told them what to say.
The conspirators also supposedly managed to torture hundreds of key Nazis into confessing to crimes which they never committed, or into framing their fellow Nazis for those crimes, and to plant hundreds of documents in Nazi files which were never discovered until after the war, and only then, in many cases, by sheer luck. Goebbels' diary, for example, was barely rescued from being sold as 7,000 pages of scrap paper, but buried in the scattered manuscript were several telling entries (as translated in Lochner, The Goebbels Diaries, 1948, pp. 86, 147-148):
February 14, 1942: The Führer once again expressed his determination to clean up the Jews in Europe pitilessly. There must be no squeamish sentimentalism about it. The Jews have deserved the catastrophe that has now overtaken them. Their destruction will go hand in hand with the destruction of our enemies. We must hasten this process with cold ruthlessness."
